The chemosensitivities of 26 lung adenocarcinoma tissues were compared to those of 110 gastric adenocarcinoma tissues, using the in vitro succinate dehydrogenase inhibition (SDI) test. Tumour tissues obtained at surgery were exposed to five different anticancer drugs: carboquone (CQ), adriamycin (ADM), mitomycin C (MMC), cisplatin (DDP) and 5-fluorouracil (5-FU). The lung adenocarcinomas showed a statistically significant reduction in succinate dehydrogenase (SD) activity, compared to the gastric adenocarcinomas, after exposure to each drug. The chemosensitivity was defined as a reduction in SD activity to 50% of control or less. Lesser degrees of reduction in SD activity were defined as drug resistance. The sensitivity rates to ADM, MMC and DDP, respectively, were significantly higher in the lung than in the gastric adenocarcinomas. Tumour cells from 22 (84.6%) of the 26 lung adenocarcinoma tissues showed a sensitivity to more than three drugs, whereas the rate was only 46.4% (51/110) for the gastric adenocarcinomas. The rate of resistance to all the drugs tested was 3.8% (1/26) for the lung adenocarcinomas, in contrast to the 20.9% (23/110) seen with the gastric adenocarcinomas. Thus, while adenocarcinomas of the lung and stomach both show clinical resistance to anticancer agents, the chemosensitivity of the lung tissues is greater. In the light of these observations, attention must be directed to improving specific drug delivery systems.