This study was done to detect the difference in T-cell function between healthy preterms and normal full terms and the effect of sepsis on T-cell function in preterms using the difference in response of these cells to P.H.A stimulation.We found that there is no difference in T-cell function between healthy preterms and full terms and there is no effect for variations in gestational age and birth weight on the function of these cells, but there is marked reduction in the function of these cells associated with sepsis and the reduction increases as sepsis severity increases.