ABSTRACT: Sequential observations of lymphocyte sub-populations, response to mitogens, level of plasma human chorionic gonadotropin (HCG), and detection of HCG on the surface of T cells were carried out to study immunological functions in pregnant women early in the fetomaternal relationship. T cell counts determined by the standard method decreased gradually between 5 and 10 weeks of pregnancy but no decline was observed when determined by the modified methods. Minor fluctuation of B cell populations and lymphocyte response to mitogens were within the standard value. Plasma HCG in peripheral venous blood (PVB) increased markedly until 10 weeks of gestation and then declined. T cell poulation in parauterine venous blood (PUVB) was lower than that in PVB. The presence of HCG on the surface of T cells was demonstrated in maternal lymphocytes but not those of control subjects.