Patients presenting with advanced or metastatic colorectal carcinoma were randomized to treatment with either 5-Fluoro-Uracil + leucovorin or 5-Fluoro-Uracil + interferon alpha-2-a. Longitudinal series of Carcino-embryonic-antigen were obtained and analyzed with a dynamic model in seven of 16 patients had CEA levels above 10 ng ml-1. Clinical evaluation based on X-ray, ultrasound and CT-scans showed a partial response to the given treatment in three of the seven patients. The dynamic model consistently estimated the maximal accumulated cell kill (approximately three logs) in the patients experiencing a clinical response. The remaining four patients with CEA levels above 10 ng ml-1 showed some reduction in CEA as a consequence of the treatment cycles, although no clinical response could be established. No response was seen among the remaining nine patients with CEA levels below 10 ng ml-1.