Polyclonality of IgA production in IgA nephropathy (IgAN) was studied to elucidate the immunological aberrations in patients with this disease. The amounts of IgG, IgA and IgM and their kappa/lambda ratios in sera and supernatants of lymphocyte cultures were determined by the enzyme-linked immunosorbent assay. This method was sensitive enough to detect as little as 10% of monoclonal IgA in total IgA. The amount of IgA in IgAN was significantly increased in sera and supernatants without pokeweed-mitogen (PWM), but not changed in supernatants with PWM. There were no significant changes in the production of other immunoglobulins between IgAN and controls both in vivo and in vitro with and without addition of PWM. As far as the kappa/lambda ratios of immunoglobulins were concerned, no significant deviations were observed between IgAN and controls in any samples. These results indicated that increased IgA in patients with IgAN was polyclonal, supporting the concept of the heterogeneity of this disorder.