Physiological reactions of viridans streptococci were examined by the API-20STREP system and a selection of conventional tests. Cluster analysis of these results produced a classification similar to a taxonomic scheme based on that of Colman and Williams. The organisms could be divided into the six recognised species--Streptococcus mutans, S. bovis, S. mitior, S. sanguis, S. salivarius and S. milleri. Analysis confirmed that S. mitior and S. sanguis can be distinguished in the API-20STREP test by hydrolysis of arginine but not by dextran production. Although S. mutans, S. mitior and S. sanguis can be divided into two further subgroups, the taxonomic significance of this is unclear. With this means of classification, most organisms could be identified easily by a small number of tests. API-20STREP is convenient for performing physiological tests on viridans streptococci, but the information provided by the manufacturers in regard to identification and nomenclature is in need of revision.