3'-azido-3'-deoxythymidine (AZT) was administered orally to 8 AIDS patients at a dose of 100 mg every 6 hours for 14 days. On days 8 - 14 the patients were also given 1 g inosine-pranobex (INPX) every 6 hours. On day 7, while the subjects were taking AZT alone and on day 14 while they were receiving AZT + INPX, blood samples were obtained over a 6-hour dosing interval for measurement of AZT by a specific AZT radioimmunoassay. AZT levels on day 14 were significantly higher than the corresponding levels on day 7, resulting in a 2-fold increase of the area under the serum concentration-time curve (AUC) and a prolongation of the mean half-life of AZT (44 to 70 min) during the INPX treatment. INPX is an immunomodulatory drug with an inhibitory effect on HIV. The potential advantages of a combined treatment AZT + INPX are: 1) need for lower dose of AZT for maintaining a therapeutic anti-retroviral level; 2) a longer interval period between AZT treatments; 3) a potential to enhance immunological response resulting from INPX treatment; 4) reduced costs of care for patients.