The purpose of this study was to characterize and quantify T lymphocyte populations and to investigate T cell antigen receptor (TcR) expression in human endometrium throughout the menstrual cycle and in early pregnancy. Frozen sections of endometrium were labeled using the highly sensitive avidinbiotin method and a panel of monoclonal antibodies. CD3-positive, CD8-positive, CD4-positive, TcR alpha beta-positive, and TcR gamma delta-positive cells were demonstrated in all phases of the menstrual cycle and in early pregnancy. There were no differences between pregnant and nonpregnant human endometrium in the proportion that each T cell subpopulation formed relative to the total CD3-positive T cell population. However, significantly fewer T cells were detected in early pregnant compared with nonpregnant endometrium. Our results clarify the TcR expression by T cells in pregnant and nonpregnant human endometrium and indicate that endometrial T cells are unlikely to play a significant role in implantation and the maintenance of human pregnancy since they decrease in number considerably in the first trimester of gestation.