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White mice of 10-12 g were immunized with one of the three virus vaccines (vaccines against poliomyelitis, measles, smallpox) at various intervals. Poliovirus type II and measles virus, Edmonston strain, were labeled in tissue culture with 3H-uridine (30/uCi/ml). Smallpox virus (rabbit strain) was labeled with 3H-thymidine (30/uCi per ml) also in tissue culture. After purification and concentration of labeled poliomyelitis and measles viruses, viral RNA was isolated by double extraction with phenol, and precipitation with alcohol to which a yeast RNA-carrier was added. Isolation of 3H-thymidine-labeled viral RNA from smallpox virus was carried out by the same method with the addition of SDS to the final concentration of 1%. From the brain and lung cells of the vaccinated animals DNA was extracted by the kinetic reassociation method and hybridized with labeled viral nucleic acids. The formation of a hybrid with DNA-containing vaccine virus was controlled by chromatography in hydroxylapatite. No integration of viral and cell nucleic acids was demonstrated in our experiments, however, it cannot be ruled out completely, because this method does not detect homologous sequences if they occur in a small number of cells tested.