['At birth the fetus emerges from a sterile environment into a nonsterile one. This process is associated with activation of the fetal immune system which protects the fetus against infection in the newborn period. We conducted this study to determine whether activation of the monocyte-neutrophil system occurs in fetuses before premature birth.', 'Forty patients in premature labor with intact membranes underwent cordocentesis for research purposes. Fetal blood was analyzed with the use of flow cytometry to measure the cell surface markers CD11c, CD13, CD15, and CD67, which are associated with monocyte and neutrophil activation, and CD14 and CD63, which were used as controls.', 'Twenty-eight percent (11/40) of the infants were delivered prematurely within 72 hours of entering the study while the remainder were delivered at term. Our data clearly indicate that premature infants delivered within 72 hours had a higher percentage of CD11c, CD13, CD15, and CD67 than those delivered at term. In contrast, there were no significant differences in the percentages of CD14 and CD63.'] Is premature parturition characterized by in utero activation of the fetal immune system?