The effect of splenectomy on the ability of alveolar macrophages of young and adult rats to phagocytize Pneumococci, Types 3 and 14, andPseudomonas aeruginosawas studied. Young animals showed a significant (15%) decrease in the phagocytosis of pneumococci type 14, 4 weeks after splenectomy. This depression increased to 30% in 6 weeks' time. Such depression was also noted when young splenectomized rat alveolar macrophages were challenged withPseudomonas aeruginosabut not with type 3 pneumococci 6 weeks postsplenectomy. Three months following splenectomy in young animals, the rats were grown and they seemed to regain their normal phagocytic activity against pneumococci type 14. Adult rats also showed no alteration in their phagocytic activity against type 3 pneumococci. Autoimplantation of the spleen had a protective effect on the phagocytosis of type 14 pneumococci, and a nonsignificant effect on that of type 3. The present study postulates a modulatory role of the spleen on alveolar macrophage function. Splenectomy may cause the impairment of local lower respiratory immune function, making lungs vulnerable to specific bacterial invasion. Such splenic modulatory effect on alveolar macrophage phagocytic function seems to be age and antigen specific.