We evaluated four commercially available kits for rapid identification of Actinomyces and related species. The kits identified correctly 26 to 65% of "classical" Actinomyces strains to the species level and 13 to 49% of newly described Actinomyces strains to the genus level, thus indicating relatively poor applicability and a need to update these kits.The genus Actinomyces embraces a heterogeneous group of facultatively anaerobic, gram-positive, mainly branching rods, which occur as frequent inhabitants on mucosal surfaces but also as common opportunistic pathogens in various infectious processes (3,5,7,10,15,17). Improvements in identification methods have resulted in expanded recognition of new Actinomyces species from clinical specimens (3,4,6,8,13,18). On the other hand, some former Actinomyces species have been moved to other genera, such as Arcanobacterium and Actinobaculum (11,14). Over 80% of the problematic isolates sent to our reference laboratory turn out to belong to the genus Actinomyces, indicating that clinical microbiology laboratories need updated information on these slowly growing, asporogenous rods. Recently, Sarkonen et al. published an extensive identification scheme for Actinomyces species for the purpose of clarifying their accurate identification on the basis of phenotypic testing alone (16); however, many laboratories rely solely on commercial kits because of lack of manpower and facilities. In the present study, we demonstrated the performance of four commercial test kits in the identification of "classical" (i.e., described before 1994) Actinomyces species and tested the reactivity of some newly described species not yet included in the databases.The RapID ANA II system (Remel, Lenexa, Kans.) consists of 10 wells (including dehydrated reactants for 18 biochemical reactions) and the Rapid ID 32 A system (bioMérieux, Marcyl'Etoile, France) consists of 32 cupules (including 29 containing dehydrated test substrates) for the testing of anaerobic bacteria. The RapID CB Plus system (Remel) consists of 18 wells (including 4 for carbohydrate utilization and 14 for single-substrate enzyme tests) for the testing of coryneform and related bacteria. The BBL Crystal ANR ID system (Becton Dickinson Microbiology System, Cockeysville, Md.) consists of 30 wells (including 17 wells with fluorescent substrates, 12 wells with chromogenic substrates, and a fluorescence control well); the assay includes tests for fermentation, oxidation, degradation, and hydrolysis of various substrates.A total of 21 reference strains from international culture collections and 86 clinical Actinomyces strains (65 of infectious origin and 21 from human resident floras) were included in this study. All strains were previously identified by conventional methods (16) or by the ARDRA (PCR-restriction fragment length polymorphism) method (8) or by both. The organisms were revived from frozen (Ϫ70°C) stocks, grown twice in subcultures on brucella blood agar, and incubated anaerobically at 35°C for 3 to 4 days before testing was con...