One of the following procedures was carried out in 112, 200-gr, Sprague-Dawley rats: (A) sham operation, (B) hemisplenectomy, (C) 90% splenectomy, (D) total splenectomy and autologous spleen transplants in either omental, muscle, or subcutaneous pouch after total splenectomy, and (E) homologous splenic transplants after splenectomy. The rats were challenged intravenously with sheep, cat and guinea pig erythrocytes 6 wk, 3 mo, and 6 mo postoperative. All totally splenectomized rats had significantly reduced primary responses, regardless of whether fragments of splenic tissue had been grafted or not. There was initial lowering of primary response in partially splenectomized animals but they responded significantly better than totally splenectomized rats. After 6 mo the primary response in the hemisplenectomy group was not significantly different from the nonoperated control group. The primary response in the 90% splenectomy group had improved significantly when compared to the total splenectomy group and normal controls. Total splenectomy and partial splenectomy did not appreciably reduce the ability to mount secondary and teritary responses.