Using a radioimmunoassay for human leukocyte interferon (IFN alpha), pharmacokinetic studies were carried out in twelve cancer patients given sequential intramuscular injections of Hu IFN alpha 2. Even though individual monitoring of serum IFN titers emphasized, for a given dose, marked quantitative variations of the observed maximum concentrations, their mean values were found to be dose-dependent (358 +/- 167 U/ml at 30.10(6) U and 1044 +/- 599 U/ml at 100.10(6) U doses). Comparison with bioassay results showed that IFN activities measured in sera were of the same order of magnitude as those calculated from radioimmunoassay standard curves. Data obtained from this series on observed peak time, half-life value and serum concentrations were consistent with those reported by the other groups using recombinant leukocyte interferon in clinical trial. Therefore, radioimmunoassay is an useful method for routinely assaying IFN alpha used either as antitumour or antivirus agent because of its high sensitivity (4 U/ml) and its simplicity.