Abstract: This paper deals with the distribution of the disodium salt of cantharidic acid in mice transplanted with Ehrlich carcinoma. Each mouse was given 0.133/μc/ml of ³H disodium salt of cantharidic acid intraperitoneally, and killed 1, 4, 24, and 48 hrs. after injection.The cancer cells were collected from the animal, fixed in 4% glutaraldehyde and osmium tetroxide, then kept in 1/15 M phosphate buffer at pH 7.2—7.4. After centrifugation, dehydration and embedding in Epon 812, samples were cut into ultrathin sections with a LKB ultratome, double stained with leadcitrate and uranyl acetate, and observed with a DX-2 electron microscope. Autoradiographic procedures were carried out with Ilford L₄ single layer emulsion and exposed for 40—90 days at 4℃.The radioactivity of the tritium labeled compound in the blood and tissues were measured with a liquid scintillation counter. It was found that the radioactivity of the blood sample reached its maximum 2 hrs. after administration. According to the decrease of their radioactivity, the tissues were ,arranged in sequence as follows: brain, gall bladder, kidney, liver, thymus, spleen, lung, heart, stomach, mus.c.le, etc, with. brain being the highest and intestine the lowest.With electron microscopic autoradiography it was shown that the labeled compound located mainly in the nuclei and mitochondria of the cancer cells.