The early steps of Rous sarcoma virus-Bryan strain (BS-RSV) infection of genetically resistant and susceptible chick embryo cells were investigated. The methods used were the uptake of virus by cells grown in monolayer, and the ability of infected cells to form foci of transformed cells after plating on resistant and susceptible assay plates. It was found that the initial attachment of virus to either genetic type of cells did not differ appreciably. Attachment on susceptible cells was immediately followed by penetration, which was 70% complete after 1 hour. Attachment on resistant cells was not followed by penetration of the cell membrane. It was concluded that the genetic block of resistance to infection of chick embryo cells by BS-RSV occurs at the step of viral penetration of the cell membrane.