Children suffering from chronic osteomyelitis of long tubular bones were compared for the action of the immunomodulating drug tactivin injected subcutaneously and endolymphatically. The study was carried out in 57 patients staying in hospital. The patients were distributed into the following groups: 18 patients entered a group where tactivin was injected subcutaneously, 9 patients made up a group where the drug was administered endolymphatically, and a reference group included 30 patients treated by conventional methods. On admission the patients demonstrated secondary immunodeficiency characterized by the lowering of the absolute and relative T lymphocyte count, a decrease of functional activity in blast transformation of lymphocytes, and dysimmunoglobulinemia. The use of immunomodulation with tactivin whatever the route of administration promoted a more uneventful course of the postoperative period, a decrease of suppurations of the postoperative wounds, reduction of the patient's hospital stay, a decrease in the dose of antibacterial drugs required for continuous treatment, and enhancement of the efficacy of antibacterial therapy. The use of tactivin endolymphatically brings about rapid normalization of the count of T, T active and B lymphocytes, an increase of the indicated parameters to the lower limit of normal on subcutaneous injection. As for the reference group, these parameters continue a progressive lowering.