Recently, a novel class of murine type C virus (MCF), some strains of which are highly oncogenic in the AKR acceleration test, has been isolated from premalignant and malignant thymuses of AKR mice. The biology of these viruses suggested that MCFs are the product of recombination between endogenous ecotropic and xenotropic viruses and, further, that the recombination has taken place within the envelope (env) gene which encodes the surface glycoprotein (gp70) of the virion. We have compared by tryptic peptide analysis, the gp70s of four MCF isolates with the gp70s of various possible parental viruses. In addition, we have compared the tryptic peptides of the gag gene products p30 and p15 from several of these viruses. The results allow the following conclusions: (i) the gp70s of the MCF viruses are not identical to one another and are different from the gp70s of the possible parental viruses tested; (ii) the MCF virus gp70s have tryptic peptides in common with xenotropic virus gp70s as well as with ecotropic virus gp70s; and (iii) the gap region protein, p30, of the MCFs tested is identical to p30 of AKR ecotropic virus (Akv-1 or Akv-2) and distinct from p30 of xenotropic viruses, suggesting that the 5' end of the recombinant viruses is of Akv origin. The findings are discussed with respect to the possible role a recombinant virus might play in leukemogenesis in AKR mice.