Previously, we have isolated and characterized a complex glycoprotein antigen (gp600) from the rat kidney that can induce Heymann nephritis (HN) in the rat. A monospecific antibody against the gp600 was used as a probe to document the existence of cross-reactive antigens in normal rat serum. A competitive radioimmunoassay measured the concentration in normal rat serum as being 45.5 +/- 10.2 micrograms/ml (n = 17). Molecular exclusion gel chromatography of normal rat serum identified gp600 activity in three distinct peaks corresponding to the molecular weights of 150,000, 110,000 and 70,000, respectively. Soluble immune complexes of mean molecular weight 1.1 X 10(6) were formed when normal rat serum was reacted with affinity-purified 125I anti-gp600. Normal rat serum, electrophoresed in 8% SDS-PAGE gels, transblotted to nitrocellulose membrane and reacted with anti-gp600 by indirect immunoperoxidase technique, identified three to four bands in the molecular weight region of 66,000-80,000. Isoelectric focusing revealed these antigens to be anionic (pI of 4.5-5.5) in nature. We conclude that normal rat serum contains antigens that cross-react with gp600. Further, these antigens are anionic in nature and form soluble immune complexes with anti-gp600 in vitro. The relevance of these findings to the pathogenesis of HN is discussed.