The sensitivity to 1-hexylcarbamoyl-5-fluorouracil (HCFU) of 25 colorectal cancer tissues was compared with that of six antitumor drugs: carboquone (CQ), Adriamycin (ADM), mitomycin C (MMC), aclacinomycin A (ACR), cisplatin (DDP), and 5-fluorouracil (5-FU), using the in vitro succinate dehydrogenase inhibition (SDI) test. Chemosensitivity was determined to be positive when the succinate dehydrogenase (SD) activity of the drug-exposed cells, at ten times the peak plasma concentration, was decreased to below 50 percent of that of control cells on day 3 of exposure. Decrease in SD activity was remarkable in cases of exposure to HCFU, compared with six other drugs. The sensitivity rates were 32 percent for CQ, 40 percent for ADM, 24 percent for MMC, 28 percent for ACR, 32 percent for DDP, 16 percent for 5-FU, and 68 percent for HCFU. The sensitivity rate for at least one of the six drugs (CQ, ADM, MMC, ACR, DDP, and 5-FU) was 52 percent, but was 80 percent when HCFU was taken into account. Since colorectal cancer tissues are resistant to various antitumor drugs, the chemosensitivity test of HCFU should aid in determining the effects of a particular drug for an individual patient.