With increasing interest in the role of fetal programming in child and adulthood diseases, and therefore interest in the measurement of various factors at birth, it is essential to ascertain whether the factors of interest show any gestation- or parturition-associated changes. We have investigated whether mode of delivery influenced T-cell phenotype and function (CD4+) as has been described for monocytes and neutrophils. Interferon-gamma production in response to either the mitogen phytohemagglutinin or anti-CD2/CD3/CD28 F(ab')3 was significantly reduced by neonatal mononuclear cells compared with adult cells but did not differ with mode of delivery at term (normal vaginal delivery versus elective lower-segment cesarean section). Likewise, anti-CD2/CD3/CD28-stimulated IL-2 production by the neonate was lower than adult levels but did not differ with mode of delivery. The expression of common T-cell activation markers (CD25, MHC class II, CD69, CD62L, CD11a, CD44, and CD49d) was examined. Only CD62L (L-selectin) expression was significantly different, with fewer adult T cells expressing this surface antigen compared with neonatal T cells (p < 0.0003), and significantly more T cells from lower-segment cesarean section than normal vaginal delivery were positive for CD62L (p = 0.012). sCD62L levels were significantly lower in cord plasma compared with adult plasma but did not differ with mode of delivery. Thus the phenotype and function of cord blood T cells did not differ greatly with mode of delivery, but possible differences for the marker of interest should always be assessed. Furthermore, although there was no significant difference with mode of delivery for all markers, except CD62L, the variation in the normal vaginal delivery samples, as for the adults, was greater than in the lower-segment cesarean section samples, indicating that the effects of length of labor and stress at delivery may well be relevant.