In this study, we demonstrated the expression of a 170,000-Mr polyprotein in each of several McDonough feline sarcoma virus (FeSV)-transformed mink cell clones and one McDonough FeSV-transformed rat clone. This polyprotein designated McDonough FeSV P170, contained feline leukemia virus (FeLV) p15, p12, and p30 immunological determinants and shared two of its five [35S]methionine-labeled tryptic peptides with FeLV Pr180gag-pol. Both of these peptides were shown to be specific to the p30 component of Pr180gag-pol. The remaining McDonough FeSV P170 methionine-containing peptides were not represented within either FeLV Pr180gag-pol or Pr82env. Of interest, of the three peptides specific to the nonstructural component of McDonough FeSV P170, one was also represented in the 115,000-Mr polyproteins encoded by the Gardner and Snyder-Theilen strains of FeSV. These findings raise the possibility that the nonstructural components of polyproteins encoded by each of the three independently derived feline transforming viruses contained both common and unique regions. Moreover, if the sequences encoding these components are involved in transformation, as appears to be the case, our findings establish that the position of their insertion within the gag-pol region of the FeLV genome can vary among individual isolates.