A total of 20 patients with advanced colorectal cancer received recombinant leukocyte interferon-alpha A (rIFN alpha A) either chronically (group I: twice a week up to 20 X 10(6) IU/m2 i.m.) or cyclically (group II: 1-4 periods of 8 consecutive days up to 20 X 10(6) IU/m2 i.m. daily at 20-days intervals) over a period of 12 weeks. There was 1 partial response, 1 mixed response and 1 patient with stable disease, whilst 17 patients had progressive disease. Median survival was 15.5 months. Survival was significantly shorter when the extent of hepatic disease was greater than 25% (P = 0.05), extrahepatic disease was extensive (P less than 0.005), alkaline phosphatase level was greater than 2 X normal (P less than 0.02), or performance status was less than 100% (P less than 0.001). Toxicity consisting mainly of fever, fatigue, anorexia and weight loss was serious in group I and minimal in group II. Administration of rIFN alpha A led to a "short lived" augmentation of natural killer (NK) cell activity. In the cyclically treated group this was a recurrent phenomenon whereas a marked lasting depression of NK cell activity was seen in chronically treated patients. Interferon-gamma production capacity was significantly stimulated during rIFN alpha A therapy. The differences in toxicity and immunostimulatory effects between the two schedules may be of importance in the design of further studies.