Thirty patients with incurable gastrointestinal cancer were treated in a randomized clinical trial, either with combination chemotherapy (5-fluorouracil, vincristine, and methyl-CCNU) or with the same chemotherapy and the thymic extract TP-1. T-lymphocyte counts slowly and steadily, increased in the TP-1-treated patients over a period of 4-5 months, in contrast to decreasing counts in the patients treated by chemotherapy only. The changes in both groups were significantly different from pretreatment counts (P<0.01). Skin tests for primary sensitization with DNCB were converted to positive or improved after TP-1 treatment (P<0.05 against control patients). In conclusion, TP-1 had a progressively accumulating effect on T-cell lymphopoiesis and activity (skin tests), which was strong enough to overcome chemotherapy-induced immunosuppression without any appreciable side effects.