By using monoclonal antibodies, two-color immunofluorescence techniques and flow cytometry, we evaluated the surface marker phenotypes of lymphocyte subsets in cord blood samples from fetuses in the second trimester of pregnancy. The results indicate that cells of the T-, B- and NK-cell lineages as well as precursor cells can be detected in fetal blood at 18–20 weeks of gestation. At this stage of development, variable proportions of T and B lymphocytes express surface molecules, such as the CD1, CD 10, CD38, CD45RA, indicative of a precursor or ‘naïve’ state; on the other hand, the CD 5 7 molecule is not detectable on the membrane of NK and T cells, and the RO isoform of the CD45 leukocyte antigen is synthesized by a low percentage of T cells. We suggest that the observed phenotypic peculiarities of the lymphoid cells might be related to the easy induction of tolerance that occurs in the early ontogenetic stages of the immune