The studies were carried out on non-immunized or SRBC-immunized Balb/c mice (8 weeks of age) exposed to immunosuppression by a single i.p. dose (125 mg/kg) of hydrocortisone. The mice were immunized i.p. with 0.2 mL of 10% sheep red blood cells (SRBC) suspension 24 h after hydrocortisone administration. Lysozyme dimer at a dose of 20 µg/kg was administered to non-immunized mice once or four times at 24-h intervals prior to hydrocortisone injection or once 2 h prior to SRBC or four times at 24-h intervals after antigen stimulation in SRBC-immunized mice. It has been found that hydrocortisone injection to non-immunized mice decreased the percentage of immature CD4⁺CD8⁺ thymic cells with a corresponding increase in the percentage of mature CD4⁺ and CD8⁺ thymic cells. In addition, the percentage of CD3⁺ and CD4⁺ mesenteric lymph node cells was reduced. The administration of hydrocortisone (125 mg/kg) to SRBC-immunized mice decreased the number of splenocytes producing haemolytic antibodies (PFC) and discontinued the production of total and 2-mercaptoethanol resistant anti-SRBC haemagglutinins. Lysozyme dimer (20 µg/kg) administered four times to non-immunized mice prior to hydrocortisone injection reduced its suppressive action on the percentage of immature CD4⁺CD8⁺ thymic cells. Moreover, pre-treatment with lysozyme dimer potentiated the increasing effect of hydrocortisone on the percentage of mature CD4⁺ and CD8⁺ thymocytes. The strong effect was noted after a single injection of the drug. Lysozyme dimer administered prior to hydrocortisone, irrespective of the number of doses applied, increased the percentage of CD3⁺ and CD4⁺ splenocytes, and counteracted the suppressive action of hydrocortisone on the percentage of CD3⁺ lymph node cells with a corresponding increase in the percentage of CD8⁺ cells. Pre-treatment with lysozyme dimer did not change the suppressive action of hydrocortisone on the percentage of CD4⁺ mesenteric lymph node cells. Lysozyme dimer administered to SRBC-immunized mice after their exposure to pharmacological immunosuppression leads to partial restoration of PFC and their capability of producing anti-SRBC haemagglutinins. The restorative action of lysozyme dimer did not depend on the number of doses applied.