This experiment was performed to evaluate the morphological responses of 5-fluorouracil or mitomycin C on the spleen of the mice. 5-fluorouracil (60 mg/kg) or mitomycin C (400 microgram/kg) were injected subcutaneously every other day, and the animals were sacrificed at 1 week and 2 weeks following the first injection. Pieces of the tissue were taken from the spleen, fixed in 10% neutral formalin for light microscopy. The paraffin sections stained with hematoxylin-eosin, Masson-trichrome, Bielschowsky's silver impregnation or aldehydefuchsin. For electron microscopy, the tissues were prefixed with 2.5% glutaraldehyde-1.5% paraformaldehyde followed by post-fixed with 1% osmium tetroxide. The ultrathin sections stained with uranyl acetate and lead citrate. The observed results were as follows: 1. On histological study, in the mitomycin C treated group, macrophages which contain pyknotic nuclei were observed more frequently than in those of 5-fluorouracil treated group. 2. In the 5-fluorouracil treated group, positive reactions to Masson-trichrome and Bielschowsky's silver impregnation were observed in the splenic capsule and traculae at the 1 week, and weak postive stains were observed at the 2 weeks. 3. In the mitomycin C and the 5-fluorouracil group, positive staining reaction to aldehyde-fuchsin were observed in splenic capsule, trabeculae and around artery at the 1 week and 2 weeks. 4. On the ultrastructural study, distended cisternae of granular endoplasmic reticula were observed frequently at 1 week. 5. In the mitomycin C treated group, myelin figures in the lymphocytes and reticular cells were observed more frequently than in those of 5-fluorouracil treated group. From the above results, it was concluded that lymphocytes and reticular cells of the spleen were slightly damaged by 5-fluorouracil or mitomycin C, and mitomycin C seems more harmful on the spleen than 5-fluorouracil does.