In order to examine the correlation between immunoglobulin (Ig) classes of immunoglobulins in circulating immune complexes (CIC) and immunoglobulins in glomerular deposits, we have measured CIC of IgG, IgA and IgM classes in various glomerular diseases. Serum CIC were measured using a modified conglutinin binding assay (K assay) using 125I-labeled anti-γ, anti-α and anti-µ antisera. IgA class CIC were detected in more patients with IgA nephropathy than patients with non-IgA nephropathy. There was an association between the presence of IgG deposits in kidneys and the presence of IgG in CIC. Patients with IgA deposits in their kidneys also had IgA class CIC. There was also an association between the presence of IgM deposits in kidneys and the presence of IgM in CIC. These results suggest that the Ig class in CIC and the Ig class in glomerular deposits were correlated, and that in IgA nephropathy the mesangial IgA deposits may be derived from IgA class CIC.