Deposition of a major retroviral envelope glycoprotein, gp 70, in renal glomeruli of ddY mice, an animal model for primary IgA nephropathy, was examined by immunofluorescence. The positive staining of gp 70 was not observed in glomeruli of our substrain of ddY mice at any ages examined using two different anti-gp 70 antisera and three different staining conditions, whereas deposition of IgA, IgG and IgM was manifest in mice aged over 40 weeks. As a control, NZB x NZW F1 mice from 4 months of age onwards showed severe glomerular deposition of gp 70, in keeping with previous reports. Thus, it appears that gp 70 deposition may not be sine qua non for the pathogenesis of IgA nephropathy of all substrains of ddY mice.