Serial determinations of plasma carcinoembryonic antigen (CEA) levels were assayed for 34 patients with advanced gastrointestinal cancer undergoing 5-fluorouracil and mitomycin C combination chemotherapy. 21 patients (62%) had an objective response and the CEA level was decreased in 12. After chemotherapy was initiated a rise in CEA level occurred on the average 6.5 months later, while the mean time to clinical relapse was 8 months. 5 patients (15%) had stable disease; the CEA level was decreased in 2. In 8 patients (24%) with progressive disease, the mean time for a rising CEA level was 3 months. In our view, a change in the CEA level reflects the effectiveness of combination chemotherapy in patients with advanced gastrointestinal cancer.