Immunoglobulin production by peripheral blood mononuclear cells of 27 patients of IgA nephropathy and 11 relatives was determined. In comparison with 15 healthy controls, no significant difference could be observed in both IgA nephropathy patients and relatives of the group not stimulated with PWM, but in the group stimulated with PWM a significant elevation in the production of IgA, IgG and IgM was seen in IgA nephropathy patients, while in the relatives a significant elevation in production of IgA and IgG was observed. It is speculated that immune complexes mainly IgA are the chief cause of development and progression of IgA nephropathy and that IgG and IgM are also involved. In also relatives, the presence of immunological abnormalities similar to those of IgA nephropathy patients is suggested.