Listeria monocytogenes isolates from human sporadic and epidemic cases (n l 119) and from animal cases (n l 76) were characterized by automated ribotyping and PCR-restriction fragment length polymorphism (PCR-RFLP) typing of the virulence genes actA and hly. This combination of typing methods differentiated 39 distinctive strains, each reflecting a unique combination of ribotypes, hly and actA alleles. Simpson's index of discrimination indicated a high discriminatory ability of ribotyping for both animal (0867) and human isolates (0857), which was further increased by the addition of hly and actA typing (0916 and 0904, respectively). Ribotype and hly allele data were further used to group isolates into three genetically distinct lineages. Each lineage is composed of several ribotype fragment subsets, each of which contains multiple ribotypes characterized by common ribotype fragments. To determine whether certain clones of L. monocytogenes show indications for unique pathogenic potential or host specificity, frequency distributions for five genetic characteristics (i.e. lineage, ribotype, ribotype fragment subset and hly and actA allele) were calculated for isolates from animal cases, human epidemic cases and human sporadic cases. Lineage III isolates were found less frequently in human cases (1 of 119 isolates) than in animal cases (8 of 76 isolates ; P l 0003). These results suggest the possibility of host specificity for non-primate mammals among lineage III strains. In addition, lineage I strains were found more frequently among human cases than among animal cases (P T 0001). Among the eight hly alleles observed, hly allele 1 was more common among human isolates as compared to animal isolates (P l 0002). We also identified one ribotype (DUP-1030) which was significantly more common among animal isolates (P l 0005) and one ribotype (DUP-1038 ; lineage I) which was significantly more common among human epidemic isolates as compared to human sporadic isolates (P T 0001). These findings confirm the presence of clonal groups of L. monocytogenes, which appear to be characterized by unique virulence or host specificity patterns. This study also establishes baseline data describing the genetic diversity of human and animal L. monocytogenes isolates which can be utilized in future surveillance programmes to track the emergence of new strains.