We have investigated the pattern of glycosylation of the membrane glycoproteins encoded by a polycythemic strain of spleen focus-forming virus (SFFV). These include a major species designated gp52 and its processed form which is designated gp65. The SFFV glycoproteins were found to be predominantly intracellular, although a portion of gp65 is expressed on the cell surface. gp65 was observed to be highly sialylated and resistant to digestion with endoglycosidase-H (endo-H). In contrast, gp52 was endo-H sensitive and the polyacrylamide gel electrophoresis profile of the endo-H digests suggested the presence of four glycosylation sites. Analysis of tryptic glycopeptides from gp52 by reverse-phase high-performance liquid chromatography also suggested the presence of four glycosylation sites. Glycopeptide analysis of Pronase digests of gp52 revealed two major size classes with molecular weights of 2200 and 1500, which correspond to two of the four oligosaccharide size classes reported previously for MuLV gp70's (M.C. Kemp, N.G. Famulari, P.V. O'Donnell, and R.W. Compans, 1980, J. Virol. 34, 154). Both glycopeptide size classes were sensitive to digestion with endo-H. The glycopeptide profile of gp65 was found to be very heterogeneous and the predominant form was a 2900-dalton size class. In addition a fucosyl glycopeptide of 2500 daltons was observed in gp65, but not in F-MuLV or F-MCF glycoproteins. In the presence of the sodium ionophore monensin, the processing of gp52 to gp65 was inhibited. Instead a smaller protein of about 60,000 daltons was observed, which did not arrive at the cell surface, a situation analogous to the processing and post-translational modification reported for gp52 from anemic isolates of SFFV (S.K. Ruscetti, J.A. Field, and E.M. Scolnick, 1981, Nature (London) 294, 663).