In cancer therapy, immunological disorder is one of most severe problem. Since thymic cortex is the home of T-cell proliferation and "education", thymic morphology following administration of certain drugs can be used as a parameter of immunological safety of the drug. In this study, morphology of thymic cortex, following administration of 5-fluorouracil or AG60, was studied. AG60 is a newly developed anti-cancer remedy, the compound of acriflavine and guanosine (1 : 1). ICR mice were subcutaneously inoculated with Ehrlich carcinoma cells (10(7) cells/mouse) in their inguinal areas. Each mouse in 5-fluorouracil group was injected subcutaneously with a single dose of 30 mg/kg of 5-fluorouracil every other day, and the mouse in AG60 group, with 30 mg/kg of AG60 (Taerim Pharm. Co., Seoul) every other day. The control mouse was injected with saline. The mice were sacrificed on the day after 7th injection. Tissues of thymic cortices were fixed in 2.5% glutaraldehyde-1.5% paraformaldehyde solution (0.1 M Millonig's phosphate buffer, pH 7.3), and refixed in 2% osmium tetroxide solution (0.1 M Millonig's phosphate buffer, pH 7.3). Tissue blocks were dehydrated, and were embedded in araldite mixture. For the overview-comparison, semithin sections stained with toluidine blue solution were photographed. And the typical portions were cut with ultratome, stained and observed with electron microscope. In light microscopy, thymic cortical morphology of AG60-injected mouse was similar with that of control mouse. But the cortical morphology of 5-fluorouracil-injected mouse was impressively different from those of the control or AG60 group mice. Thymocytes in the thymic cortex of 5-fluorouracil-injected mice were severely depleted. In electron microscopy, thymocytes in the thymic cortices of the control or AG60 group mice were crowded, and small groups of thymocytes were surrounded by the cytoplasmic processes of epithelial reticular cells. Mitotic figures were randomly seen. Thymocytes of 5-fluorouracil-injected mouse were naked out from the epithelial reticular cells, and were completely depleted out from the cortex composed mainly of enlarged epithelial reticular cells. Numerous microvilli were protruded from the naked thymocytes. The results were interpreted as that 5-fluorouracil induce leukopenia, and homing of lymphocytes to thymic cortex is severely depressed. 5-fluorouracil also disturb the normal protective and supportive function of epithelial reticular cells for thymocytes. Whereas the complex of acriflavine-guanosine compound (AG60) is immunologically safe, as seen in thymic cortical morphology.