To investigate the regulatory effect of aqueous extract of leaves of Ocimum sanctum on IL-2 cytokine production in vivo and in vitro, and the effect of leaves extract on general blood picture including T& B lymphocytes. For in vivo studies albino Wistar rats were treated with aqueous crude leaves extract of Ocimum sanctum for 20 consecutive days. Spleen cells were harvested and assayed for IL-2 production by using sandwich enzyme-linked immunosorbent assay (ELISA) and mRNA expression methods. For in vitro study aqueous Ocimum sanctum leaves extract = in different concentrations (25–500 μg/mL) was added into culture plates containing ConA stimulated splenocytes. To study the overall effect on blood picture, density gradient purified lymphocytes analysis and conventional methodology for total and differential leukocyte count and hemoglobin level were also done. It indicated that the rats treated with Ocimum sanctum leaves extract had significantly enhanced (P < 0.001) ability of spleen cells to secrete IL-2. Investigation in vitro also showed regulation of IL-2 production. Blood study exhibited leucocytosis and augmentation of T& B lymphocytes by 25% approximately. 4–5% increase in Hemoglobin value was also noticed. Aqueous Ocimum sanctum leaves extract may have stimulatory effect on T & B lymphocytes particularly on Th1 subset of lymphocytes as shown by enhancement in IL-2 production.