The injection of Rous sarcoma virus (RSV) into the wing web of newly hatched chicks causes a rapidly growing sarcomatous tumour which is palpable within 1 week of inoculation; and cultures of fibroblasts derived from chick embryos (CEF) and infected with RSV become rapidly transformed. Genetic studies have determined that expression of a single viral gene, designated v-src, is necessary for neoplastic transformation. This gene codes for a 60,000-molecular weight phosphoprotein termed pp60SPC , which possesses a protein kinase activity that phosphorylates polypeptides on tyrosine residues and is constitutively expressed in infected CEF cells. It has been suggested that transformation, and possibly tumorigenesis, may result solely from the consequences of this increase in tyrosine phosphorylations. The pathogenicity of RSV in chick embryos in ovo is less clear. Murphy and Rous suggested that RSV may have caused tumours in "various tissues" of "some embryos", but the subsequent studies of Milford and Duran - Reynals , as well as several other laboratories, failed to find any evidence of intraembryonic tumours in RSV-infected early embryos. The findings of Duran - Reynals , if correct, cannot be explained easily in view of our present understanding of RSV tumorigenicity. Thus, we have re-examined the interaction of RSV with the avian embryo and confirm here that RSV is nontumorigenic and non-teratogenic when microinjected into day 4 chicken embryos. In addition, we found that (1) the virus not only replicates in the embryo, but it also expresses an active src-specific protein kinase and (2) once the cells from the infected limbs are disrupted and placed in culture, they are capable of expressing the transformed phenotype after a 24-h delay.