Rauscher leukemia virus glycoprotein gp9/71 is synthesized in virus-infected cells by way of a 90,000 dalton glycoprotein precursor, termed Pr2a+b. This precursor could blaeled with radioactive glucosamine and methionine but not with fucose; whereas gp69/71 could be detected by labeling with radioactive glucosamine, fucose, or a mixture of amino acids but seemed to be deficient in methionine relative to Pr2a+b. Pr2a+b and gp69/71 were specifically precipitated by an antiserum prepared against phosphocellulose purified Rauscher gp69/71. Other virus-specific precursors, in addition to Pr2a+b, could be precipitatedby antiserum prepared against detergent disrupted virus. Neither Pr2a+b nor gp69/71 was precipitated from cell extracts by antisera to Rauscher p30. Tryptic maps of Pr2a+b and gp69]71 showed that these glycoproteins share many tryptic peptides. Pulse-chase experiments with 14C-labeled amino acids indicated that gp69/71 was not radio-labeled during the pulse-labeling period but slowly appeared during the chase incubations. Pr2a+b, however, was rapidly labeled and tended to disappear during long chases. Furthermore, two nonglycosylated viral proteins, termed pl5E and pl2E, are structurally related to Pr2a+b. Viral p15E and pl2E contained-the same methionine-containing tryptic peptide fraction as Pr2a+b as determined by ion-exchange chromatography. These results provide evidence that Pr2a+b is a precursor to gp69/71 and establish a structural and possible precursor-product relationship between Pr2a+b, pl5E, and pl2E. RNA tumor virus structural proteins are made by way of relatively large precursor polypeptides. In the avian system, p27, p19, p15, and p12 sequences are found in a 76,000 dalton precursor present in avian myeloblastosis virus-infected cells (1)*. In the murine system, Rauscher leukemia virus (RLV) p30 sequences are present in several precursors ranging in size from 65,000 to 200,000 daltons (2-4). Peptide sequences of another viral protein, termed pl5E (5)