To investigate the role of IgA-containing circulating immune complexes (IgA-CIC) in mesangial cell proliferation, human mesangial cells were cultured with sera or IgA isolates from IgA-CIC-positive IgA nephropathy patients and negative controls. A solid phase anti-C3d enzyme-linked immunosorbent assay (ELISA) was used for the detection of IgA-CIC. Sera stimulated the proliferation of the mesangial cells at both 24 and 72 hours' incubation. However, there was no difference in the proliferation of cells incubated with sera from IgA-CIC-positive patients and negative controls. IgA-CIC-positive and negative IgA isolates both stimulated proliferation of mesangial cells at 24 hours' incubation. Our data do not support the hypothesis that IgA-CIC are directly responsible for mesangial cell proliferation.