1. Splenectomy in the rat results in a marked elevation in the number of circulating leukocytes and platelets. 2. Transient suppression of the leukocytosis may result from the presence of a 10 per cent splenic implant, either free in the peritoneal cavity or enclosed in a Millipore diffusion chamber. 3. Marked and persistent suppression of postsplenectomy thrombocytosis results from the presence of a 10 per cent splenic implant, whether free or enclosed in a 0.45 µ Millipore diffusion chamber. 4. The results provide indirect evidence of a humoral substance produced by rat spleen which influences platelet levels; presumably this substance acts by suppressing thrombopoiesis.