Studies in animals and clinical experience in man have demonstrated that splenectomy leads to increased susceptibility to infection with encapsulated bacteria. Splenic tissue has an excellent ability to regenerate, even when implanted into subcutaneous tissue or the abdominal cavity. These implants, however, do not protect against bacterial challenge despite the fact that a number of other functions can be restored. We therefore studied the ability of residual splenic tissue to protect against challenge following subtotal splenectomy in Sprague-Dawley rats. Subtotal splenectomy was performed on 48 animals in which approximately 75% of the spleen was removed and left with a branch of its normal blood supply; 48 animals underwent total splenectomy and 48 had sham operations. Six months after surgery the groups were challenged intravenously with type 25 pneumococci to determine the LD50 for each group. Animals that had undergone subtotal splenectomy were more resistant to pneumococcal challenge than were asplenic animals, but they were not as resistant as normal animals. In addition, there was marked delay in death in the animals with subtotal splenectomy as compared with asplenic animals. Thus residual splenic tissue after subtotal splenectomy appears to confer some degree of protection against pneumococcal challenge.