The extent of homology between the translation products of the HZ2 strain of feline sarcoma virus (HZ2-FeSV) and the Abelson murine leukaemia virus (A-MuLV) was examined immunologically and biochemically. Antiserum prepared against the v-abl-encoded determinants of the A-MuLV polyprotein P120gag-abl was also found to precipitate specifically the 98K mol. wt. HZ2-FeSV protein (P98gag-abl). The basis for this immunological crossreactivity was indicated by the findings that the two proteins had at least six [35S]methionine-containing tryptic peptides and at least eight [35S]methionine-containing chymotryptic peptides in common. Each of the two proteins also had tryptic and chymotryptic peptides which were unique. Both proteins were associated with tyrosyl kinase activities which exhibited some similar biochemical properties in vitro. However, the HZ2-FeSV-associated activity was much more sensitive to competitive inhibition by nucleoside and deoxynucleoside diphosphates than was the A-MuLV-associated activity. These results suggest that, while the gag-abl translation products of these two independent isolates of transforming retrovirus are highly related structurally and functionally, the differences in structure contribute to differences in enzyme activity. Further comparative studies of these two proteins should play an important role in determining their roles in induction of two different types of malignancy: lymphosarcoma in the case of the A-MuLV protein and fibrosarcoma in the case of the HZ2-FeSV protein.