With the aim of potentiating the anti-tumour effect of interferon in metastatic malignant melanoma by concomitant inhibition of suppressor T cells, oral cimetidine (histamine-2 receptor antagonist) medication was added to interferon (HuIFN-alpha(Le] therapy in a series of 20 patients. While no objective tumour responses were recorded with interferon treatment alone administered intramuscularly or intratumorally, six patients had objective tumour regressions on subsequent combined therapy. Five out of eight patients with metastases confined to skin and subcutaneous tissue had complete tumour regressions while one patient with skin and lung metastases achieved an extensive partial regression of the skin tumour and a complete roentgenological regression of the lung metastasis. Three additional patients attained a stable disease status for prolonged periods of time. Histopathological examinations confirmed disappearance and/or degeneration of melanoma cells and demonstrated a marked lymphocyte infiltration in tumour sites of the patients with objective tumour regression.