The retroviral envelope glycoprotein, gp70 was measured in the serum of New Zealand Black (NZB) and SWR mice and the progeny of their crosses. The serum gp70 values segregated to "NZB-like" and "SWR-like" levels in these mice. A complex mechanism determined the inheritance of NZB-like serum gp70 levels. We found that the factors determining the expression of this retroviral protein were independent of the genes (Nzv-1 and Nzv-2) that determined the expression of infectious xenotropic virus. Autoimmune disease, including immune deposit nephritis could be dissociated from the degree of expression of serum gp70. By contrast, presence of circulating immune complexes and anti-DNA antibody did correlate with the development of nephritis in these crosses. A significant correlation was found between high grade expression of serum gp70 and the presence of lymphomas in these mice.