Avian sarcoma virus (ASV)-induced tumor cells that are derived from regressing neoplasms contain about 75% less pp60src kinase activity than do progressively-growing tumor cells. Paradoxically, the polyadenylated RNAs extracted from 'regressor' cells found in regressing neoplasms are much more productive than those from progressively-growing tumors in a reticulocyte lysate in vitro translation system, as determined both by time course and concentration curve studies. Electrophoretic analysis of polypeptides corresponding to known ASV proteins further suggests that the translation product from the mRNAs of regressing tumor cells contains much more viral protein than does that derived from progressively-growing sarcoma cells. The identity of two of these viral proteins was confirmed by immunoprecipitation of the respective translation products, using a tumor-bearing rabbit serum with specificity for pp60src and Pr76gag precursor polypeptides, and showed that much more of pp60src and Pr76gag were produced by the mRNAs from regressor than from progressor cells. We further found that ASV genes including genomic and subgenomic species (8.6, 5.0 and 3.2 kb) were present in greater amounts in regressing as opposed to progressively-growing tumor cells. These results support the notion that viral RNAs are accumulated in 'regressor' tumor cells.