Isoniazid (INH), antituberculous drug with immunomodulatory properties, have been described as lupus-like syndrome inducer. The development of autoimmunological phenomena results from immunoregulatory disturbances. The goal of this work was to determine the influence of INH on selected parameters of the immune response in vivo and in vitro. In vivo (in B6AF1 mice) the influence of long-term treatment on primary humoral response and cellular response was evaluated. Drug dose was 25 mg/kg. In vitro (using peripheral blood of volunteers) the influence of INH on mitogen induced proliferation, metabolic activity of granulocytes and production of angiogenic cytokines by diverse subpopulation of mononuclear cells was examined. The concentrations tested were 0.5 mg/ml, 5 mg/ml and 50 mg/ml. No effect of INH could be demonstrated on the production anti-SRBC antibodies nor on the cellular response in mice. In vitro INH added to the cell cultures increased PHA and ConA stimulated proliferation. The chemiluminescence of human granulocytes increased in the presence of INH. Drug enhanced production of angiogenic cytokines by human lymphocytes CD4+ and suppressed angiogenic activity of CD8+ cells. The results suggest that INH has strong immunomodulatory properties which may explain its involvement in pathogenesis of lupus-like disease.