The sensitivity of Sindbis virus gradually decreases during serial passages in the presence of 1-adamantane carbonic acid amide. After 4 passages, viral reproduction was not suppressed even by subtoxic concentrations of the inhibitor. Sensitive Sindbis strain principally differed from the resistant one: the latency of reproduction of resistant virus was 2 h longer than that of the sensitive strain. Cells infected with the sensitive and resistant strains of Sindbis virus contained the major structural proteins (C, El+E2), their precursors pE2, p130, p90, and p78 protein, which probably represent a nonstructural virus-specific protein nsP3. The synthesis of virus-specific polypeptides and RNA of resistant variant is resistant t 0 the inhibitor during incubation.