Studies in animals and clinical experience in patients have demonstrated that splenectomy may lead to an increased susceptibility to infection. The infections are usually caused by encapsulated bacteria such as penumococcus. It has been shown in a variety of experimental animals that autotransplanted splenic tissue is capable of regenerating into implants that are microscopically indistinguishable from normal spleen and of restoring a number of normal splenic functions. The response to intravenous challenge with Streptococcus pneumoniae, type 25, was therefore studied in control, asplenic, and autotransplanted Sprague-Dawley rats. Despite previous observations that a number of immune functions can be restored in this animal model by autotransplanted splenic tissue, the present study indicates that splenic tissue autotransplants do not restore the ability to resist intravenous pneumococcal challenge.