Thirty-five eligible patients with disseminated malignant melanoma received intramuscular recombinant leukocyte interferon (IFN-rA), 50 X 10(6) U/m2 three times weekly (TIW) for an intended duration of 12 weeks concomitant with daily oral cimetidine, 1,200 mg/d in four divided doses. For all study participants, the median survival time was six months. Among 21 "good risk" patients (performance score [PS] 0, 1 and no prior chemotherapy), we observed seven partial regressions (33%). Six patients had stability of disease (29%), seven had immediate disease progression, and one discontinued treatment after two doses without tumor evaluation due to side effects. Times to disease progression of five patients with regressions of soft-tissue disease were 2.1, 3.3, 3.5, 3.7, and 4.3 months. Two patients had partial regressions of lung nodules for 2.0 and 3.8 months. We observed one regression among 14 "poor risk" patients (PS 2, 3, or prior chemotherapy). A 46-year-old woman with prior treatment had a partial regression of soft-tissue disease for 4.1 months. Four "poor risk" patients achieved disease stability, and nine progressed immediately. Leukopenia (WBC count less than 4,100/microL) affected 21 (66%) of 32 patients with WBC count data. The median count was 3,100/microL; range, 1,300 to 8,400/microL. We detected two cases of mild thrombocytopenia (100,000 and 120,000/microL). Other noteworthy toxicities included moderate-to-severe nausea (34%), anorexia (63%), and fatigue (80%). All patients experienced myalgias. Twenty patients had dosage decreases during the first cycle, and 14 of the 16 patients remaining on study after the first cycle required dosage reductions. The overall response rate is similar to our prior studies with IFN-rA as a single agent using TIW doses of 50 X 10(6) U/m2 and 12 X 10(6) U/m2 among 31 and 30 patients, respectively.