Both variants of HIV-1 reported in the literature: slow/low and rapid/high types, were detected among the strains isolated from the subjects examined in 4 foci of HIV-1 infection in the south of the RSFSR and Byelorussia. All the 17 strains isolated in the southern RSFSR foci belonged to the slow/low type and had a low and unstable replication potential in donor peripheral blood mononuclear cells and in MT-4 cell line. All of them were isolated from subjects with asymptomatic infection and from children with initial clinical manifestations of the disease. Only one strain isolated in Byelorussia belonged to the rapid/high type. Its replicative activity was very similar to that of the classical HIV-1--HTLV-IIIB strain. Long-term (up to 7 months) propagation of slow/low strains did not result in any increase of their replicative activity. The capacity to form syncytia was found not only in the rapid/high type strains but also in the majority of slow/low strains under study.