Sensitivity of 142 human large bowel malignancies to gastroenteropancreatic hormones (VIP, glucagon and pentogastrin) and calcitonin was studied using in vitro adenylate cyclase reaction of tumor. At least 40-55% of the tumors proved hormone sensitive. Heteroresponse (reaction to calcitonin) was most characteristic for colonic tumors whereas weak reaction to VIP and glucagon-for rectal neoplasms. A certain relationship was established between adenylate cyclase reaction to hormone stimulation, on the one hand, and peculiarities of tumor (degree of cell differentiation) and the body (gender), on the other. In patients who survived over 4 years, tumor adenylate cyclase had initially been more sensitive to hormone stimulation than in those who died over that period. It is concluded that tumor adenylate cyclase reaction to hormone stimulation is quite a reliable test for evaluating hormone sensitivity of large bowel tumors and, possibly, for choosing hormonal therapy.