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The International Working Group on Mycobacterial Taxonomy has undertaken a series of cooperative studies to standardize and establish reproducibility of tests that are useful for classifying and identifying mycobacteria. To date 25 techniques have been examined, and 5 of these met our rigorous criteria for reproducibility and differential power. The properties determined by these tests are urease activity, pigment production, tolerance to 5% NaC1, hydrolysis of Tween 80, and fl-galactosidase activity.The International Working Group on Mycobacterial Taxonomy (IWGMT) is an informal confederation of investigators who have undertaken a series of cooperative studies into the taxonomy of the genus Mycobacferiurn (3, 6). The members also serve in an advisory capacity t o the Subcommittee on Mycobacteriurn of the International Committee for Systematic Bacteriology. The data derived from the cooperative studies have not only contributed to a definition of species (clusters) of the mycobacteria, but have identified those properties that are most definitive of species and, thus, of the greatest potential determinative value. However, these investigations were conducted under a permissive philosophy, with each participant free t o select the specific technique to be employed for determining a given property. The principles and logistics of the IWGMT studies have been discussed previously (6).To contribute t o greater standardization of methods applied to the systematic bacteriology of mycobacteria, a new series of studies has been undertaken to evaluate the reproducibility of specific techniques. This is a report on the first IWGMT pilot cooperative study of standardization of techniques for use with slowly growing mycobacteria. MATERIALS AND METHODSSelection of tests for study. The tests were chosen on the basis of prior cooperative studies of the IWGMT that dealt with slowly growing mycobacteria. These cooperative studies were concerned with the taxonomy of scotochromogens of Runyon group I1 (6), non-photochromogens of group 111 (Meissner et al., J. Gen. Microbiol., in press), photochromogens of group I (Tacquet et al., manuscript in preparation), and members of the M. tuberculosis-M. bouis complex (Kleeberg et al., manuscript in preparation). Characters were defined as of potential systematic value if over 85% of the strains of one or more cluster recognized in these studies exhibited the character and if over 85% of the strains of one or more of the remaining clusters failed to exhibit the character.A total of 69 characters, derived from 25 tests, met the criteria defined above. Participants in the prior cooperative studies were asked to provide details of the techniques they had employed, and from these responses a working manual was developed in which the techniques were described in detail. 412