Renal biopsies from 21 patients with minimal change nephrotic syndrome and mesangial IgM deposits were investigated by means of fluorescence, light and electron microscopy; elution of tissue-bound antibody; and fixation of heterologous (guinea pig) complement. In 12 patients complement and IgM deposits were associated and, in 4 of these, electron dense deposits conforming to immune complexes were detected in mesangia. Antibody elution and heterologous complement fixation studies in tissues suggested that such immune reactants may represent interaction of complement-fixing antibody and antigen. Long-term follow-up studies are needed to determine the clinical relevance of IgM deposits in minimal change nephrotic syndrome.