Actinomyces strains were tested for evidence of bacteriocin-like antagonism. Prevalence of inter-strain antagonism was low, and, when present, zones of inhibition were generally small and poorly defined. Strains of A. viscosus were very susceptible to bacteriocin-like inhibition by S. mutans. Prevalence of inhibition was high, and zones of inhibition tended to be large and well defined. Inhibitors produced by S. mutans were generally heat stable, nondialyzable, and sensitive to proteolytic enzymes. Individual strains of A. viscosus could be differentiated on the basis of sensitivity patterns to a battery of S. mutans strains.