In vitro MTT assay was applied for examining chemosensitivity with 50 thyroid cancers; 47 papillary cancers, 1 follicular cancer, 1 anaplastic cancers, and 1 medullary cancer. Anticancer drugs examined were 5-fluorouracil (5-FU), cisplatin (CDDP), adriamycin (ADM), etoposide (VP-16) and ifosfamide (IFO) (active type). Their efficacy was as follows: 5-FU 0%, CDDP 40.9%, ADM 0%, VP-16 5.3%, IFO 48.5%. Anaplastic cancer and medullary cancer showed no sensitivity to all tested drugs. We compared the chemosensitivity between cancer tissue and normal tissue. CDDP was more effective in cancer tissue than in normal tissue. Chemosensitivity of CDDP depended on the drug concentration but 5-FU did not. CDDP and IFO might be useful against human differentiated thyroid cancer.