The chemosensitivities of 41 poorly differentiated gastric cancer tissues were compared with that of 16 well differentiated tissues, using the in vitro succinate dehydrogenase inhibition test. These human tissues obtained at the time of surgery were exposed to six different antitumor drugs: carboquone (CQ), adriamycin (ADM), mitomycin C (MMC), aclacinomycin A (ACR), cisplatin (DDP) and 5-fluorouracil (5-FU). The chemosensitivity was determined as positive when the succinate dehydrogenase (SD) activity of the drug exposed cells was decreased to below 50% of that of control cells, on day 3 of exposure. Decrease in SD activity was remarkable in the poorly differentiated tissues, compared to the well differentiated tissues, exposed to ADM, MMC, DDP and 5-FU. The sensitive rates were higher in the poorly differentiated tissues than in the well differentiated tissues, against all six antitumor drugs. Sixty-three per cent of the poorly differentiated tissues were sensitive to more than three antitumor drugs, in an identical tissue, but the rate was only 19% in the well differentiated tissues. The resistant rates to all drugs tested were 20% in the poorly differentiated and 31% in the well differentiated tissues. This would indicate that patients with a poorly differentiated gastric cancer will probably show a better response to antitumor drugs, compared to those with a well differentiated type.