Recovered avian sarcoma viruses, whose sarcomagenic information is largely derived from cellular sequences [Wang, L.-H., Halpern, C.C., Nadel, M. & Hanafusa, H. (1978) Proc. Natl. Acad. Sci. USA 75, 5812-5816], produce the transforming protein p60src in infected cells, in amounts comparable to the amount found in cells transformed by standard strains of avian sarcoma virus. Though displaying some virus-specific differences in electrophoretic mobility, p60srcs from these viruses are similar to those of other avian sarcoma virus strains by the criteria of (i) antigenicity, (ii) partial proteolysis mapping, and (iii) association with protein kinase activity. We also find that p60sarc, a protein present in normal cells at a low level, is associated with a protein kinase activity, and thus it too is similar by the above criteria to p60src of avian sarcoma virus. Possible causes for the pathogenicity of p60src are discussed in light of these similarities.