A co-operative taxonomic study has been performed on slowly growing nonpigmented mycobacteria (Run yon's group 111). Phenetic data on 89 strains, studied in 18 laboratories, were collected and analysed by a numerical taxonomic method. A variety of immunological properties, lipid analyses and measures of pathogenicity were analysed independently to establish correlation with numerical classification. Mycobacterium gastri, M . nonchromogenicum, M . terrae, M . avium and M . xenopi were recognized by almost all participants as distinct species. Mycobacterium novum was considered to be synonymous with M . terrae. A clearcut distinction could not be made between M . avium and M. intracellulare; the majority of participants in the study recommend that M . intracellulare be reduced to a synonym of M . avium. A minority of authors cannot agree with this proposal.

A co-operative numerical taxonomic analysis of slowly growing scotochromogenic mycobacteria of Runyon's group I1 with no restrictions on numbers of characters contributed by individual participants, or on techniques employed to derive these characters, is reported. Of four scotochromogen clusters defined in this study, three were of named species (Mycobacterium Jlavescens, M. gordonae and M . scrofulaceum) and one, a small cluster of four strains, may represent a new species. Results from pooled data are compared to results obtained with data from individual participating laboratories. Immunologic data were treated separately and showed good correlation with the numerical analyses.

An open-ended study has been initiated by the International Working Group on Mycobacterial Taxonomy to accommodate strains of slow growers that do not belong to well-established or throughly characterized species. Numerical taxonomic analysis of the data derived from the first series of cultures examined in this study has permitted preparation of expanded descriptions of Myco bacterium simiae and M. szulgai, and, to a lesser extent, M. asiaticum. The analysis also provides some support for revival of the name "M. shimoidei," which is presently without standing. Additional clusters have emerged which may represent new taxa; one of these includes a strain labeled "M. paraffinicum," a name which is also now without standing.Many commonly encountered species of slowly growing mycobacteria have been characterized in a series of cooperative taxonomic studies carried out by the International Working Group on Mycobacterial Taxonomy (IWGMT) (8,21,22). The rapid growers were characterized in two additional IWGMT studies (6, 12), as were members of the "rhodochrous" taxon (4) which is now recognized as Rhodococcus. In the original numerical taxonomic (NT) studies of the slow growers, some strains failed to aggregate with any of the major clusters and were too few in number to permit assignment to new taxa. To provide a mechanism for continuous introduction of unusual or rarely encountered strains into the mycobacterial taxonomic framework, the IWGMT has initiated a new effort, designated the Open-Ended Study of Slowly Growing Mycobacteria. Accession and detailed study of strains is an ongoing process; as new clusters emerge, their descriptions will be reported and, when appropriate, new species will be proposed. This is the first such report from the IWGMT open-ended study. MATERIALS AND METHODS Selection of strains.The data base for this study was derived from cultures meeting one of the following three criteria.(i) Reference marker strains. One culture was selected from a central position in each of the clusters that corresponded to named species in the three prior cooperative studies on slow growers (8,21,22). These 13 cultures, which are identified in the NT matrix (see Fig. l), were not redistributed for further examination; the data previously derived from them were transferred into the computer file reserved for the OpenEnded Study.(ii) Unclustered strains from the prior studies. These strains either appeared in an isolated position or were linked to one or two other strains in one or another of the previously reported studies. These cultures also were not redistributed, but the existing data were simply transferred into the new file.(iii) New accessions. Submission of strains for study was not limited to members of the IWGMT. Anyone wishing to submit a strain for consideration was required to forward it to one of the designated IWGMT transmitters

A co-operative taxonomic study has been performed on slowly growing photochromogenic mycobacteria (Runyon Group I) and closely related organisms. Phenetic data on 54 strains, studied in seven laboratories, were collected and analysed by numerical taxonomic methods. Immunological properties and phage susceptibility patterns were analysed independently to establish correlation with numerical classification. Mycobacterium gastri, M . kansasii and M . marinurn appeared as distinct well-defined clusters and the serological and phage data supported the resolution of these three species. A table of definitive properties is presented. Two strains each of M. simiae and M . asiaticum formed a loose cluster which was clearly separated from the previously nzentioned three species; the small number of strains examined precluded the establishment of a list of definitive properties of these two species. It is concluded that the Runyon Groups, which provided a practical though arbitrary basis for establishment of a series of co-operative studies, have served their purpose and should now be supplanted by classification and nomenclature based on species.

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