The authors analysed CD4+ subset populations and particularly subset killing in order to evaluate the presence of antilymphocyte antibody against CD4 + subsets in patients with IgA nephropathy (IgA N). This study was performed in 45 patients with IgA N, 30 patients with other forms of glomerulonephritis and 30 healthy controls. CD4 + CD4 + CD45R- and CD4 + CD45R+ cells in the peripheral blood were counted by a flow cytometric analysis, and those cell killings were analysed by microcytotoxic assays. The percentage of circulating CD4 + CD45R + cells was significantly decreased in IgA N, and the percentage of CD4 + CD45R+ cell killing was significantly elevated in IgA N compared with other groups. There was a significant negative correlation between the percentage of CD4 + CD45R + cells present in IgA N patients’ peripheral blood lymphocytes and the killing CD4 + CD45R + cells by the same patients’ serum. Both a depletion of CD4 + CD45R+ cells in peripheral blood lymphocytes and an elevation of CD4 + CD45R+ cell killing correlated with the grade of mesangial proliferation in patients with IgA N. However, there were no correlations in other clinicopathological indices. These results suggest that low levels of antilymphocyte antibodies against CD4 + CD45R+ cells were present in patients with IgA N, who showed a depletion of CD4 + CD45R+ cells in the peripheral blood. These antibodies were strongly associated with the elimination of CD4 + CD45R+ cells and the proliferation of glomerular mesangial cells in patients with Ig