The phenotyping of T-cell subsets and T cells at different stages of activation was performed using a panel of monoclonal antibodies in samples from normal pregnant women at different stages of gestation and in the cord blood of neonates. The data obtained from pregnant women showed a slight decrease in the total number of T cells at the beginning of pregnancy, whereas there was a clear increase in 4F2-positive lymphocytes after a few months of gestation. No significant increase in Class II-positive lymphocytes was observed in normal pregnant women in comparison with adult healthy women. The data from neonates revealed a clear decrease of OKT3- and OKT4-positive cells and an increase of 4F2-positive cells in comparison with control subjects. These data indicate that alerted, but not fully activated, lymphocytes are present in the circulation of both the mother, after the first months of pregnancy, and the neonate. This finding reinforces the concept that during pregnancy there is an activation of certain immune components rather than a general depression of the immune system.