In this experiment, side effects of three anticancer drugs (5-fluorouracil, mitomycin C and AG60) on the structure of spleen were histologically studied. Each of ICR mice was inoculated with 1X10(7) Ehrlich carcinoma cells subcutaneously in the inguinal area. From next day, 0.2 ml of saline solution, 5-fluorouracil (30 mg/kg), mitomycin C (400 micorgram/kg), 5 mg/kg of AG60, or 30 mg/kg of AG60 (acriflavine (1) : guanosine (1) composition, Taerim Pharm. Co., Seoul, Korea) were injected subcutaneously every other day. The day following the 7th injection of anticancer drugs, each mouse was injected with a single dose of 0.7 microCi/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. Tissue blocks were washed, dehydrated, embedded and cut into 6 micrometer-thick sections. 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 deveoped 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 mm2) were observed and calculated. The results were as follow: 1. On histological study, in the spleen of mitomycin C treated group, vacuoles containing pyknotic nuclei were observed more frequently than in those of 5-fluorouracil treated group. Whereas in the AG60 treated group, morphological changes of the spleen were not observed. 2. On autoradiographic study, most of the labeled cells were located near the marginal zone of the spleen. 3. On autoradiographic study, number of the labeled cells of normal control, experimental control, 5-fluorouracil-treated, mitomycin C-treated, AG60 (5 mg/kg)-, and AG60 (30 mg/kg)-treated groups were 214.0+/-56.87, 235.7+/-59.69, 331.0+/-67.20, 137.0+/-33.48, 124.6+/-34.28, and 64.9+/-16.26, respectively. 4. In the mitomycin C treated group and AG60 (5 mg/kg or 30 mg/kg) 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. But in the 5-fluorouracil treated group, number of the heavy labeled cells were observed more frequently than in those of the normal and experimental control groups. From the above results, DNA synthesis, in the cells of the marginal zone were more active than in the cells of the white pulp or the red pulp. And mitomycin C and AG60 may suppress the DNA synthesis of the splenic tissues. Especially, AG60 does not results any histological defect on the splenic tissues. These result suggest that AG60 is expected as one of most effective anticancer drugs.