The GIX antigen, which is expressed on the surface of thymocytes of certain mouse strains, is an antigenic determinant of the major envelope glycoprotein of murine leukemia virus (gp70). Although GIX is expressed in some mouse strains that appear to be free of virus, the antigen can also be induced in GIX- mice by infection with particular murine leukemia viruses (termed GIX+). We have investigated the envelope gene products from two closely related viruses that differ in their GIX phenotype. Analysis of the envelope protein precursors by polyacrylamide gel electrophoresis and endoglycosidase treatment indicated that the GIX+ viral protein contained six oligosaccharide chains, whereas the GIX- viral protein contained seven. The observed differences in gel electrophoretic mobilities and glycopeptide profiles of the respective glycosylated envelope gene cleavage products (gp70) may be accounted for by the presence of an additional oligosaccharide chain on the gp70 of the GIX- virus. No differences between the apparent molecular weights of the nonglycosylated product of the envelope gene (p15E) were detected. These results suggest that the GIX- virus codes for an extra glycosylation site relative to the GIX+ virus, and this oligosaccharide chain is present both on the envelope gene precursor (Prenv) and on the major cleavage product (gp70). Recent nucleotide sequence analyses of selected RNase T1 oligonucleotides from the genomes of viruses that differ in GIX phenotype have similarly suggested that there may be a correlation between the GIX- phenotype and an extra glycosylation site [Donis-Keller, H., Rommelaere, J., Ellis, R. W. & Hopkins, N. (1980) Proc. Natl. Acad. Sci. USA 77, 1642-1645]. The results of these two different approaches raise the possibility that the presence of an additional oligosaccharide chain on gp70 may, either directly or indirectly, mask the expression of the GIX antigen on the surfaces of thymocytes and virus-infected cells.