Objective:To investigate the effect of human Leptin on the proliferation of human circulating T lymphocytes in vitro and the influence of Leptin on the action of PHA on the proliferation of human circulating T lymphocytes.Methods:PBMC were isolated from hepartinized venous blood of normal donors by density-gradient sedimentation over Ficoll-Hypaque and cultured in the 1640 complete medium.T lymphocytes were then obtained by depleting the monocyte and B cell populations.Cells were then cultured in the 1640 complete medium for 56 h in the presence of PHA.The proliferative effect was assessed by means of incorporation of thymidine.TdR was added and cultured for 16 hours.Cells were then processed in the harvester to measure the incorporation of [()~3H] thymidine.Radioactivity was determined by liquid scintillation counter.Results:The results showed that Leptin alone did not affect the proliferation of T lymphocytes,but it could enhance the effect of PHA on the proliferation of T lymphocytes at the concentrations of PHA from 2-8 ?g/ml.A dose-response studies of T lymphocyte proliferation showed that the maximal effect of Leptin were observed at 10 nmol/L.Leptin did not affect the proliferation of T lymphocytes when the concentrations of PHA were over 8 ?g/ml.Conclusion:The studies demonstrated that Leptin alone did not affect the proliferation of lymphocytes,but it could enhance the action of PHA on cell growth in a dose-dependent manner.