This study was designed to evaluate the clinical tolerance to multiple IM injections of rDNA-produced human alpha-2 interferon (IFN) (Schering-Plough 30500) in patients with solid tumours. IFN was administered in escalating IM doses in separate groups of patients daily for 14 days and then twice weekly for a further 10 weeks. The dosage levels were 1, 3, 10, and 30 million U/injection. Subjective toxicity could be divided into two types, acute and chronic. The acute reactions took the form of an influenza-like syndrome consisting in chills, rigors, headache, tremor, nausea, vomiting, and myalgia. These symptoms were dose-related but tachyphylaxis developed with continued dosing. The chronic toxicity consisted of malaise, lethargy, fatigue, anorexia, and confusion. These symptoms were not so dose-dependent and tended to become more severe with prolonged treatment. Objective toxicity consisted of myelosuppression and liver dysfunction. Granulocyte counts below 1.0 X 10(9)/l were seen in three patients at the 30-million-U level, with platelet counts less than 100 X 10(9)/l in two of these. Elevation of the liver enzymes were seen in all five patients treated at 30 million U, but returned to normal after 1 week without IFN in all but one patient. A tolerable dose (IM) for phase II/III studies lies between 3 and 10 million U for daily scheduling and between 10 and 30 million U for twice-weekly injections.