The quantitative study of adsorption, elution, and penetration of oncogenic polyoma virus (PV) labeled with 3H-thymidine under the influence of beta-propiolactone-inactivated Sendai virus (SV) was carried out in a permissive culture of mouse embryo fibroblasts and in two nonpermissive cultures of human embryo and chick embryo fibroblasts. In adsorption of PV on permissive and nonpermissive cells SV was found to reduce the amount of the adsorbed virus if used before the adsorption and to have no effect on adsorption upon reverse order of treatment of the cells with the two viruses. SV facilitates elution with 5 mM EDTA of PV adsorbed on both permissive and nonpermissive cells. Under the influence of SV, PV could be eluted from the surface of the permissive cells within 1 hour after adsorption and from the surface of the nonpermissive cells within 5-6 hours. Three hours after PV adsorption on permissive cells, both treated and untreated with SV, most of PV DNA (approximately 60%) was found in the nuclear fraction. At the same time, in the nuclear fraction of nonpermissive cells, treated or untreated with SV, the amount of PV DNA was insignificant (about 13%). The fraction containing large granules isolated from the cells which had been in contact with SV was found to contain approximately 50% of 3H-thymidine label.