The gag-linked transformation-specific proteins (polyproteins( of PRCII, Fujinami, and Y73 avian sarcoma viruses have been compared by tryptic peptide mapping. In addition to shared gag peptides, PRCII polyprotein p105 and FSV polyprotein p140 were found to have seven methionine-containing and five cysteine-containing tryptic peptides in common. These represent the majority of the non-gag peptides for each virus. In contrast, no overlap was detected with the non-gag peptides of the Y73 polyprotein p90. Examination of the tryptic phosphopeptides of p105, p140, and p90 labeled by their associated protein kinases gave similar results. Although the major phosphopeptides of p105 and p140 comigrated, they were distinct from the phosphopeptide of p90. Three classes of transformation-specific proteins can now be identified among known avian sarcoma viruses. After the pp60src of Rous sarcoma virus and B77 virus, the proteins of PRCII and Fujinami virus form a second class and Y73--currently the only representative--characterizes the third. Despite their structural differences, these viruses may share a common mechanism of transformation, effected by their associated protein kinases.