Bacillus cereus strain Socransky 67 and Streptococcus mutans strain Ingbritt were grown overnight in complex medium and then in fresh medium flasks for 5 h. The bacterial cells in the medium were centrifuged and resuspended; 4-14C-progesterone or 4-14C-testosterone was added, and the samples were incubated for 2 h at 37 degrees C in a shaking water bath. The metabolites were analyzed with column and thin layer chromatography and radioautography and quantified by liquid scintillation counting. On the basis of the metabolites found it was concluded that B. cereus strain Socransky 67 contains 5 alpha-steroid hydrogenase, and 3 beta-, 17 beta- and 20 alpha-hydroxysteroid dehydrogenases and probably also steroid hydroxylases, and that S. mutans strain Ingbritt contains 5 alpha- and 5 beta-steroid hydrogenases, and 3 alpha-, 17 beta- and 20 alpha-hydroxysteroid dehydrogenases. The metabolic activity of B. cereus is several times higher than that of S. mutans. We suggest that the greater ability of B. cereus to metabolize progesterone and testosterone is probably due to its growth milieu in the gingival sulcus, where it is nearer to the gingival tissue.