The sera of 37 patients with IgA nephropathy (IgA NP) were assayed for levels of antibodies specific for the Fab fragment of homologous IgA, and the values obtained were compared to antibody levels in a panel of 26 normal volunteers. IgG antibody levels in IgA NP patients were significantly elevated over those of the controls (P less than 0.01); at the same time IgM anti-Fab alpha levels were significantly decreased when compared to the control panel (P less than 0.01). There was no correlation of antibody levels of either isotype with levels of circulating immune complexes; however, IgM antibody levels of IgA NP patients showed a significant negative correlation with severity of renal insufficiency.