In this experiment, side effects of two anticancer drugs (adriamycin and CP -2) on the structure of spleen were histologically studied. Each of ICR mice was inoculated with 1 x10 7 Ehrlich carcinoma cells subcutaneously in the inguinal area. From next day, 0.2 ml of saline solution, adriamycin (2 mg/kg) or CP -2 (30 mg/kg) were injected subcutaneously every other day. The day following the 7th injection of adriamycin or CP -2, each mouse was injected with a single dose of 0.7 micro Ci/gm of methyl -3 H -thymidine (25 Ci/mmol, Amersham Lab., England) through tail vein. Seventy minutes after the thymidine injection, animals were sacrificed, and splenic tissues were collected and fixed in 10% neutral formalin. Deparaffinized sections were coated with autoradiographic emulsion EM -1 (Amersham Lab., England) in the dark room and dried, and were kept in a light -tight box. The sections were exposured for 5 weeks in the dark room, and were developed in D -19 developer. The number of the labeled cells in the areas of the white pulp, the red pulp and the marginal zone (mean number of labeled cells per 0.21 mm 2 ) were observed and calculated. In the spleen of adriamycin treated group, vacuoles containing pyknotic nuclei were observed frequently. Whereas in the CP -2 treated group, morphological changes of the spleen were not observed. The number of the labeled cells of normal control, experimental control, CP -2 treated and adriamycin treated groups were 240.3 +/-53.28, 252.3+/- 58.24, 216.7 +/-55.17 and 45.4 +/-15.46, respectively, and most of the labeled cells were located near the marginal zone of the spleen. In the adriamycin treated group, labeled cells containing a few silver grains of 3 H -thymidine were observed more frequently than in those of the normal and experimental control groups. From the above results, adriamycin and CP -2 may suppress the DNA synthesis of the splenic tissues. Especially, CP -2 does not results any histological defect on the splenic tissues. These result suggest that CP -2 is expected as one of effective anticancer drugs.