The protein composition of type-D oncovirus HEp-2, isolated from cell-free medium of continuous human HEp-2 cell line, has been investigated using electrophoresis on gradient polyacrylamide gels with sodium dodecyl sulfate (SDS). Labeling with 14C-amino acids revealed five viral polypeptides with molecular weights of 70 000 (gp70), 27 000 (p27), 19 000 (p19), 15 000 (p15), 12 000-10 000 (p12-10). The 70 000 dalton protein was shown to be the only glycoprotein by incorporation of radioactive glucosamine. A polypeptide with molecular weight of 78 000 has been specifically precipitated from pulse-labeled type-D oncovirus producing HEp-2 cells with goat anti Mason-Pfizer p27 serum. This protein was shown to be gag gene-coded polyprotein precursor (Pr78gag) of the major virus polypeptide p27. Pulse-labeled HEp-2 and Mason-Pfizer infected Tu 197 cells were rinsed, lysed, clarified and precipitated with goat anti Mason-Pfizer p27 serum. In both cases Pr78gag was detected.