The purpose of this study was to assess the effect of a low-dose rituximab (RIT) at < 375 mg/m(2) on B cells in the spleen and peripheral blood. Five renal transplant recipients received a single dose of RIT at 10, 15, 35, 150, or 300 mg/m(2) 3-13 days before transplantation. One patient who received the same immunosuppressive regimen except for RIT was also enrolled as a control. Splenectomy was performed at the time of transplantation in all patients. The B-cell count in the peripheral blood was analysed with a fluorescence-activated cell sorter using anti-CD19 antibodies, and the B cells in the spleen were analysed by immunohistochemistry using anti-CD20 and -CD79a antibodies. All but one dosage (10 mg/m(2)) of RIT completely eliminated B cells from the circulation within 30 days. Immunohistochemical examination of the spleen showed a marked reduction of B cells in the white pulps in all five recipients compared with that in the control patient. The observations in this study indicated that RIT has a potent effect of depleting B cells in the spleen and peripheral blood at low-doses of < 375 mg/m(2).