Fourteen children underwent splenectomy for congenital spherocytosis, splenomegaly, or thrombocytopenia. The patients were studied twice before the operation, three times during the first postoperative month, and one year later. A transitory rise in neutrophils and serum IgA was seen postoperatively; there was a modest but long-lasting increase in lymphocytes and a marked elevation of eosinophils. An immediate decline in serum IgM concentration was observed only in patients with an uncomplicated postoperative course, but one year after splenectomy the average IgM concentration had decreased by 23%. The in vitro lymphocyte transformation response to a panel of mitogens and antigens fell in the immediate postoperative period but was largely normalized 10 days postoperatively, except in the youngest of the patients who had repeated infections following the splenectomy. One year postoperatively the transformation response and the number of T- and B-lymphocytes in the blood were normal.