Sixty-two facultative Actinomyces strains isolated from dental plaque were tested for the production of bacteriocin-like inhibitory effects by a deferred antagonism method. When incubated anaerobically, all isolates produced identical inhibitory patterns against 15 indicator organisms, but under microaerophilic conditions, little inhibitory activity was observed. Activity was not evident after anaerobic incubation on a medium buffered by 0.5% (w/v) calcium carbonate. Gas-liquid chromatographic analyses of agar blocks removed from the inhibitory zones indicated that, compared with microaerophilic conditions, anaerobic incubation encouraged production of high concentrations of lactic and succinic acids, and the concomitant fall in the pH was probably responsible for the inhibitory effects.