A comparison of the chemosensitivity of well-differentiated human gastric cancer tissues was made between histological venous invasion positive (v(+)) and negative (v(-)) tissues, using the succinate dehydrogenase inhibition (SDI) test. These tissues obtained at the time of surgery were exposed to six anticancer drugs: carboquone (CQ), adriamycin (ADM), mitomycin C (MMC), aclacinomycin A (ACR), cisplatin (DDP) and 5-fluorouracil (5-FU). Chemosensitivity was judged to be positive when the succinate dehydrogenase (SD) activity of the drug exposed cells decreased to below 50% of that of control cells. Decrease in the SD activity was more apparent in the v(-) tissues than in the v(+) tissues, exposed to the anticancer drugs and in particular to ADM (P less than 0.01), MMC (P less than 0.02) and DDP (P less than 0.05). The sensitivity rates to all six anticancer drugs were lower in the v(+) tissues. The resistance rates to all drugs tested were 0% in the v(-) tissues, but 21% in the v(+) tissues. In light of these observations, patients with gastric cancer of the well differentiated type and the histological venous invasion, will probably show a less positive response to cancer chemotherapy.