Abstract: The influence of tumor necrosis factor-α （TNF-α） on the proliferation of FRTL-5 cells has not been certified. In the present study tetrazolium salt （MTT） colorimetry was used to study the effects of TNF-α on the proliferation of FRTL-5 cells. When the cells were cultured in 5H （TSH free） medium, both proliferative and cytolytic effects of FRTL-5 cells induced by TNF-α （200—20 000 U/ml） were not found. However, in 6H medium （containing TSH）, the proliferative responses of the cells to TSH were moderately inhibited by TNF-α in the levels from 200 U/ml to 20 000 U/ml, and there were no differences between the different dose groups. It is suggested that the inhibitory effects of TNF-α on FRTL-5 cells may be mediated by the inhibition of TNF-α on TSH.