A medium for detecting colonies of Actinomyces viscosus and Actinomyces naeslundii in dental plaque samples was developed. The medium (CNAC-20) contains 20.0 mug of 3CdSO4-8H2O per ml of Columbia CNA agar base. Laboratory strains of A. viscosus grew on CNAC-20 in characteristic round, white, smooth, opaque colonies. Increasing the cadmium concentration impaired the growth of some A. viscosus strains. Stock strains of A. naeslundii and A. israelii grew in colonies of similar white, opaque morphology. The few strains of other gram-positive plaque bacteria that grew on CNAC-20 had colonies easily distinguished from those of A. viscosus. Most of the bacterial strains freshly isolated from Actinomyces-like colonies on CNAC-20 that had been inoculated with human dental plaque samples were found to have cultural characteristics consistent with previous descriptions of A. viscosus or A. naeslundii. CNAC-20 may facilitate investigations into the relationship of microaerophilic Actinomyces with the etiology of dental diseases.