Clinical (101) and collection (26) strains of gram-positive anaerobic cocci were examined in conventional tests and pyrolysis mass spectrometry (PMS). Numerical classifications based upon conventional test reaction patterns (CTRPs) and PMS showed 27 and 22 clusters, respectively. Cross-tabulation of cluster membership in the two classifications showed excellent correlation, with the combined classifications showing clear groups corresponding to the currently recognised species Peptostreptococcus anaerobius, P. heliotrinreducens, P. hydrogenalis, P. indolicus, P. lactolyticus, P. magnus, P. micros and Peptococcus niger. Strains of P. prevotii and P. tetradius clustered together in a heterogeneous group of saccharolytic organisms. However, strains previously identified as P. asaccharolyticus were divided into three distinct groups in PMS, two of which differed only in indole-associated pyrolysis products. A further four groups and several single-member clusters were distinct from these species. PMS data supported the validity of identification by pre-formed enzyme profiles and confirmed that Hare group III is synonymous with P. hydrogenalis, Hare group IV with P. magnus, and the "ADH group" with P. vaginalis. There is clearly a need for a taxonomic revision of the genus Peptostreptococcus, which probably encompasses several generic groups.