Leptin is a hormone secreted by the adipocytes that contribute to the control of energy balance, and circulating leptin levels reflect the amount of adipose tissue in the body, helping to regulate food intake and energy expenditure. Since it has been shown that human milk contains immunoreactive leptin, which is identical to intact human leptin, we decided to investigate the possible presence of immunoreactive bovine leptin in different kinds of common commercial milk. To determine the presence or absence of immunoreactive leptin in bovine milk for human consumption, 81 samples (66 commercial pasteurized milk and 15 artificial formulae for new-born babies) of the most common Spanish commercial types of milk were studied. All samples were evaluated before and after centrifugation, and leptin levels were measured by RIA. Leptin was detected in all samples and RIA standard curves were not perturbed when centrifuged and non-centrifuged milk replaced the buffer. Mean values of leptin in full-cream, semi-skimmed and skimmed samples, were: 5.7+/-0.3 ng/ml, 4.1+/-0.1 ng/ml, 3.7+/-0.1 ng/ml (significantly different). Leptin values were reduced after centrifugation. A significant correlation was observed between leptin levels and lipid content (p<0.0005, r=0.67) while no correlation was observed with respect to carbohydrate and protein levels. Interestingly, some preparations of infant formulae present very high leptin values reaching up to 18.9 ng/ml. In conclusion, leptin is present in significant and variable concentrations in edible commercial bovine milk, with higher concentrations in infant formula.