Formaldehyde-fixed Staphylococcus aureus and monospecific antiserum to gp70, the major envelope glycoprotein of murine leukemia virus, were used to immunoadsorb gp70 from Nonidet P40 extracts prepared from surface-radioiodinated murine cells. The labeled gp70 molecules in these cells were linked to a protein of approximately 15,000 daltons via native disulfide bonding. Prior treatment of cells with the reversible, bifunctional, crosslinking reagent dimethyl-3,3'-dithiobispropionimidate, followed by immunoadsorption and two-dimensional diagonal electrophoresis, revealed apparent homodimers and homotrimers of the 85,000-dalton complex. Identical treatment of purified type C RNA tumor virus from murine cells also revealed homodimeric and homotrimeric species, demonstrating similar self-associating tendencies of this glycoprotein in both intact virus and the plasma membrane of nonproducing murine cells. One cross-linked product consistently detected on the surfaces of murine cells was not present after crosslinking of a representative strain of murine leukemia virus.