Recombinant interferon alpha-C (rIFNα-C, Interpharm), is a new type of alpha-interferon that has a specific activity of 1-2×109 units/mg protein. The pharmacokinetics of rIFNα-C were studied in 11 patients with metastatic renal-cell carcinoma. A total of 10 million units IFNα-C were injected intramuscularly and the serum level of IFN was evaluated up to 72 h post-administration. Measurable IFN concentrations appeared in the serum as early as 0.5 h, and levels peaked at 4-6 h (Cmax=53.2±4.6 units/ml). Relatively high levels persisted for 24 h and declined thereafter with an apparent half-life of 3-4 h. The mean area under the serum-concentration curve (AUC) was 1,259±145 units h ml-1, indicating good bioavailability of the preparation from the intramuscular injection.