Two separate proteins coded for by the Friend strain of the spleen focus-forming virus (SFFV) have been characterized: a 52,000 dalton glycoprotein which shares determinants with the gp70's of MCF viruses and a 45,000 dalton protein that contains F-MuLV p12 and p30 determinants. The 52K env-gene related protein is present in the cytoplasm as well as on the surface of all cells infected with all known strains of SFFV. While related to the gp70 of MCF viruses, it differs from it in that it does not contain group or interspecies determinants, it does not enter into extracellular fluid and/or virus particles, and it contains tryptic peptides which are not contained in the gp70's of either MCF or ecotropic viruses. The 45K gag-gene related protein is synthesized by only some strains of SSFV, can be found in both glycosylated and phosphorylated forms, and enters into the extracellullar fluid and/or virus particles released from cells infected with F-MuLV pseudotypes of SFFV. The two SFFV encoded proteins can be distinguished from one another by specific antisera as well as by their translation in vitro from two different size classes of SFFV RNA. Studies are in progress to determine the role that these proteins might play in the induction of rapid erythroleukemia.