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Two contact-inhibited "revertant" cell lines were isolated from an SV40-transformed mouse 3T3 cell line (SV-3T3) after exposure to 5-fluoro-2'-deoxyuridine . Revertant cells resembled 3T3 cells morphologically and grew to saturation densities which were similar to those of 3T3 cells ; however, revertant cells readily formed both single and multinucleated giant cells in confluent cultures . SV40 virus was rescued from revertant cells by fusion with permissive monkey cells . The rescued virus transformed 3T3 cells with the same efficiency as wild type virus, and produced transformed colonies which were phenotypically similar to those produced by wild type virus . The revertant cells also resembled normal 3T3 cells in that they contained higher quantities of sialic acid than SV-3T3 cells . An inverse correlation was found between the saturation density of cells and their sialic acid content . Collagen content, however, of revertant cells was similar to that of SV-3T3 cells . The data presented suggest that the property of contact inhibition in revertant cells is related to the sialic acid content of the plasma membrane and that changes in sialic acid content of transformed cells are not directly specified by the viral genome .