A case of hypocomplementemic membranoproliferative glomerulonephritis was studied during remission of nephrosis induced by high doses of corticosteroids. Hepatitis B surface antigen (HBsAg) and immune complexes were detected in serum and glomeruli. Anti-hepatitis-B surface antibody, undetectable in serum by conventional radioimmunoassays was identified in circulating immune complexes (CIC). On two occasions, improvement in renal function coincided paradoxically with an extreme increase in serum HBsAg levels as well as with marked elevation of CIC. We suggest that, as previously observed in animal models of glomerulonephritis, extreme antigen excess may inhibit glomerular deposition of immune complexes.