We examined whether the retroviral envelope antigen, gp70, is a major nephritogenic antigen in ddY mice, a murine model of spontaneous mesangial glomerulonephritis associated with IgA and IgG deposition. Immunofluorescence microscopy revealed that the mesangial gp70 deposition increased with age in mice over 24 weeks old, as did the IgG and IgA deposits. Immunoelectron microscopy demonstrated the reaction products of gp70 superimposed on the electron dense deposits in the mesangial matrix. Various amounts of serum gp70 were detected in mice as young as 12 weeks without any apparent increase with age. There was no correlation between the serum level of gp70 and the extent of the glomerular gp70 deposition, whereas mice with heavier IgA deposition had higher mean levels of serum IgA. The absorption test demonstrated that significant amounts of serum gp70 composed immune complexes in 40 week-old ddY mice developing glomerulonephritis; however, this bound form of gp70 was not observed in 12 week-old mice without glomerulonephritis. Systemic examinations by immunofluorescence staining showed that gp70 was mainly localized in various lymphoid tissues. These findings suggest that the gp70 antigen, mostly derived from lymphoid cells, may circulate as immune complexes and accumulate in the mesangial area, thus contributing to the development of glomerulonephritis in these mice. In addition, the pathogenic role of the increased IgA production in these mice was discussed.