ABSTRACT: We have made an effort to determine whether or not there is any change in subpopulations of lymphocytes in normally pregnant and preeclamptic pregnancies using monoclonal antibody markers. Eleven normally pregnant and ten women with preeclampsia were studied, both during the third trimester and again two months postpartum, and compared to eleven age-matched nonpregnant women. Mononuclear cells were isolated from heparinized venous blood. One million cells were treated with each appropriate antibody (Ortho-mune OKM1, OKT3, OKT4, OKT8, OKT11, OKIa), and then reacted with FITC-antimouse IgG and examined by flow cytometry and/or fluorescence microscopy. No significant differences between these three groups were noted in the OKT3, OKT4, OKT8, OKT11, or OKIa cellular populations. The OKM1 population was significantly decreased in the third trimester of normal pregnancies but not in the preeclamptic pregnancies. No significant differences were found 2 months postpartum.