Aberration of IgA-bearing B lymphocytes in patients with IgA nephropathy has been investigated. Twelve patients with IgA nephropathy demonstrated a marked increase of IgA-bearing lymphocytes in peripheral blood, while ten patients with chronic proliferative glomerulonephritis without mesangial deposition of IgA showed normal amounts of IgA-bearing lymphocytes. The increase of IgA-bearing lymphocytes reflected that of IgA-producing lymphocytes, since lymphocytes obtained from patients with IgA nephropathy restored a high percentage of IgA-bearing cells in vitro after treatment with trypsin. Quantitation of IgA-bearing lymphocytes in peripheral blood is a useful method for screening of patients with IgA nephropathy.