The effects of a novel antitumor antibiotic polyactin A (PA) on in vitro IL-2 production and IL-2 responsiveness of human lymphocytes were investigated. The results show that PA in a concentration range from 0.01 to 100 micrograms/ml obviously augmented in vitro IL-2 production, IL-2 receptor expression and responsiveness to IL-2 of human lymphocytes in the presence of PHA, and that these enhancing effects of PA were dose-dependent. The results demonstrated the potentiating effect of PA on cellular immunity and suggested the suitable use of PA in clinical treatment of tumors. The possible application of PA to the treatment of immune disorders involving deficiency of IL-2 production and/or IL-2 responsiveness of lymphocytes is also considered.