Cord blood mononuclear cells (MNC) were isolated from 20 normal full-term newborns. These MNC were preincubated with either 50, 100, or 200 micrograms/ml Thymostimulin or without Thymostimulin. The interleukin-2 (IL-2) and gamma-interferon (gamma-IFN) production, cytotoxicity, and lymphoproliferation and IL-2 receptor (Tac) expression were all significantly increased after Thymostimulin treatment. For evaluation of the in vivo effect, two combined-immunodeficiency patients defective on the thymic level, one with progressive BCG infection, and one with DiGeorge syndrome were used. Before Thymostimulin treatment, the patient's MNC did not produce sufficient amounts of IL-2 and gamma-IFN. The cytotoxicity and lymphoproliferation were also low. After Thymostimulin treatment, the IL-2 and gamma-IFN production, cytotoxicity, and lymphoproliferative response were enhanced. These results suggest that Thymostimulin may be beneficial in the clinical treatment of primary cellular immunodeficiency. The improved immune reactivity including cytotoxicity and enhanced IL-2 and gamma-IFN production in the Thymostimulin treatment also indicates that there may be a beneficial effect on the combination of chemotherapy and Thymostimulin.