Thirteen patients with malignant tumors were entered into a phase I trial with recombinant DNA human alpha 2 interferon (IFN alpha 2). The patients were given I.M. escalating doses of IFN alpha 2 ranging from 1-10(6) to 200-10(6) IU with a 72 hours washout between injections. In the majority of the patients, subjective symptoms were noted: fever, headache, chills, nausea, myalgias. Asthenia, anorexia, drowsiness appeared after the highest doses and disappeared without any sequellae. Leucopenia and thrombopenia were seen in 11 out of 13 patients. Hepatocellular toxicity was observed in 9 cases. Cardiac and vascular functions were not impaired by IFN alpha 2. The pharmacokinetic studies showed a maximum serum concentration between 4 and 6 hours after injection and the peak value was directly proportional to the dose. No neutralizing INF alpha 2 serum factor was detected during the treatment. The peak value for serum beta 2 microglobulin occurred 48 hours after and the N.K. activity was variably modified by IFN alpha 2 injections. A major clinical response was observed in 1 case, a minor response in 3 cases and a stabilisation of the disease in 4 cases.