Six clones of mouse hybridomas producing highly specific antibodies to the vaccine LSc 2ab strain of poliomyelitis virus type I have been derived. The clone 10C5 antibodies were used for intratypic serodifferentiation of 75 poliomyelitis virus type I strains from the collection of the Institute of Poliomyelitis and Viral Encephalitides of the USSR Acad. Med. Sci. which had been isolated in the USSR from children with paralytic poliomyelitis in 1950-1983. General large-scale vaccination campaigns for children using live poliomyelitis vaccine (SLV) had started in 1959. Among 12 strains isolated in 1950-1958, only 2 strains could be neutralized with clone 10C5 antibody. These 2 strains were evidently of the vaccine origin since they had been isolated in 1957 in Leningrad where at that time first large-scale trials of SLV had already been initiated (Smorodintsev, A.A. in: "Live Vaccine against Poliomyelitis", Leningrad, 1960, 42-60). In the period after the beginning of large-scale vaccination campaigns (1959-1983), 11 out of 26 strains and 25 out of 36 strains isolated in areas of the moderate and hot climate, respectively, showed antigenic relationship with the vaccine LSc 2ab strain. The portion of vaccine-like strains isolated in different years beginning since 1959 was sufficiently stable comprising about half of the total number of poliomyelitis virus type I strains examined.