Objective:To investigate the optimal combination dose of IL-12 and IL-2 for antitumor activities of human PBMC.Methods:By the resting or low dose (10 IU/ml) IL-2 activated the PBMC were incubated with different concentrations of IL-12 and (or) IL-2,then proliferation and cytotoxicity of PBMC was determined by ³H-TdR incorporation and LDH release assay respectively.Results:①5～50 IU/ml IL-12 alone did not cause significant proliferation of PBMC,but synergized with low-dose IL-2 to enhance proliferation in a IL-12 dose-dependant manner,and prolonged the duration of proliferation response.②PBMC stimulated with IL-12 markedly increased cytotoxic activity against both NK-sensitive K562 cell and NK-resistant Raji cell.The cytotoxicity of activated PBMC incubated with 10 IU/ml IL-12 and 10 IU/ml IL-2 achieved the maximum which was 72.0±4.2% and 65.4±5.8% against K562 and Raji cell,respectively,exceeded the sum of both cytokines alone.Conclusion:Human PBMC pretreated with low dose IL-2 in vitro then induced by 10 IU/ml IL-12 and 10 IU/ml IL-2 can emerge powerful,long-lasting proliferation and the most effective antitumor activity.