Thirteen patients with metastatic malignant melanoma received interferon alpha-2a (Roferon-A) and vinblastine. The interferon dosage was increased from 3 x 10(6) IU to 9 x 10(6) IU daily in 10 weeks and thereafter 9 x 10(6) IU was administered three times weekly intramuscularly. Vinblastine (0.075-0.15 mg/kg) was given every third week intravenously. One of the ten evaluable patients had partial remission (PR) (11%) for 10 months. The diseases was stabilized (NC) in three patients (30%) for 3, 6 and 9 months. Progression (PD) occurred in six patients. The treatment time varied from 5 weeks to 44 weeks. The median survival time from the beginning of this combination treatment was 5 months. The most common side-effects were fever, fatigue, loss of taste, weight loss and neutropenia. The mitogen response to phytohemagglutinin and purified protein derivative of tuberculin decreased in all patients. The response to concanavalin A decreased less and began to increase again in the patients with PR and NC. The natural killer cell activity in PD patients decreased more than in the patients with PR and NC. The ratio of T4/T8-positive cells was restored in PR + NC patients but rose in PD patients indicating a difference in the immunomodulatory effect of the combination or of the advanced disease itself on T-cell function in PD patients. This combination of daily interferon and vinblastine did not prove to be effective in melanoma. The depression of immunological functions, which was more marked in patients with PD, might indicate that vinblastine in this combination counteracts the immunostimulatory effect of interferon.