CNAC-20 agar was evaluated to determine its usefulness as a differential medium for strains of Actinomyces viscosus and Actinomyces naeslundii. Human and primate plaque specimens, in addition to stock strains of actinomyces and other oral bacteria were cultured on CNAC-20 agar. Plaque isolates were made from colonies judged to be actinomyces or non-actinomyces on the basis of morphology. Verification of these isolates was made with fluorescein labeled antisera specific for A. viscosus and A. naeslundii.