We have assessed the expression of leptin receptor on human umbilical cord blood leucocytes using flow cytometry, by a specific antibody, in 12 normal newborns (seven females and five males). The expression was significantly (P < 0.000 1) higher in polymorphonuclear cells (10 ± 6%) as compared to rnonocytes (23 ± 4%) and lymphocytes (16 ± 4%). There was no correlation between the weight or the sex of the newborn and the effect of leptin or the expression of its receptor. Future studies should ascertain the type of leptin receptor, its function and the role of this cytokine in cord blood cells.