Rimantadine and its structural analogs, i. e. amide of 1-adamantane carboxylic acid (AACA) and 1-adamantane acetic acid amide, were shown to be able to inhibit reproduction of Sindbis virus in culture Vero cells. AACA had the maximum antiviral activity. Subcultures of the initial sensitive population of Sindbis virus in the presence of AACA led to formation of mutants resistant to AACA as well as to rimantadine, adamantane acetic acid amide and ammonium chloride. The Sindbis virus population was heterogenous in sensitivity to AACA, which was evident from isolation of separate clones with various levels of sensitivity to the above mentioned compounds from the population. It was found that reproduction of the AACA sensitive and resistant strains of Sindbis virus differed: the latent period of the resistant strain was 2 hours longer than that of the sensitive strain. The same effect was observed in the comparative study on synthesis of the virus-specific RNA.