Twenty-four patients with histologically proven metastatic malignant melanoma were included in a Phase II trial of human DNA recombinant interferon (rDNA IFN alpha 2). They were given 10 X 10(6) IU of IFN alpha 2 subcutaneously three times a week until progression of disease or major intolerance developed. Twenty-two patients were evaluable for toxicity and response. General manifestations of intolerance were seen in all the patients. Hematologic toxicity was seen in six patients and therapy had to be interrupted in one patient. Mild liver toxicity was seen in most patients after 2 weeks of treatment. These manifestations disappeared within 2 weeks after treatment was discontinued. A partial response was seen in four cases lasting 2, 4, 4, and 5 months, respectively. There were two complete responses (one skin, one lymph node metastasis) lasting 20 and 6 weeks, respectively. These results indicate a potential role for rDNA IFN alpha 2 in treating patients with metastatic malignant melanoma. However, further trials are required to determine the optimal dose and schedule of administration and modalities of combination.