The peripheral blood of 27 women in their third trimester of pregnancy and of 16 control subjects was studied for total WBC counts and total numbers and percentages of T and B lymphocytes, including quantitation of the major immunoglobulin subtypes of the B lymphocytes. Although significant differences were found for percentages of total lymphocytes, T Lymphocytes, and B lymphocytes, the absolute numbers varied only slightly between the 2 study groups. A higher percentage and a higher absolute number of IgG-bearing B lymphocytes were found among pregnant women than among controls. It is concluded that significant quantitative alterations in circulating T and B lymphocytes do not occur in the third trimester of pregnancy; therefore, the concept of impaired cellular immunity, which has often been suggested to occur in this setting, is not supported. A review of the literature on T and B lymphocytes in human pregnancy is presented.