1. To quantify compensatory spleen growth after reduction in splenic mass and to determine the effect of age and vascular supply on remnant growth, we examined adult and weanling rats for remnant regeneration and adult rats for host protection against bloodstream pneumococcal challenge at intervals up to 180 days after partial resection or autograft implantation. 2. In adults, subtotally resected spleens with remnant blood flow and segmental anatomy otherwise intact (eutopic remnants) displayed prompt but relatively limited gains in remnant weight and nucleated cell number, and growth after 60 days was minimal; survival after pneumococcal challenge was directly related to initial remnant weight and unrelated to length of postoperative interval. Splenic autografts (ectopic remnants), by comparison, underwent necrosis early after devascularization, recovered initial remnant weight slowly, fell far short of restoring original cellularity, and even 6 months after implantation barely attained initial size; moreover, host protection against pneumococcaemia was no different in rats with autografts than in those with no residual spleen. Remnants in situ (both partial resection and whole spleens) grew more vigorously in weanling than adult rats. 3. The results suggest that splenic regeneration after either partial resection or implantation is much more limited than previously suggested, at least in disease-free rats, and is regulated by size and integrity of the initial remnant spleen, adequacy of vascular inflow, and age of the host. Whereas eutopic remnants provide protection against blood-borne pneumococcal challenge in proportion to initial residual size, ectopic remnants even with prolonged maturation fail to display immunoprotective function.