Human milk provides newborns with several physiologically important proteins not found in cow's milk, including bile salt-stimulated lipase (BSSL) that compensates for the reduced lipolytic capability of the newborn intestine. We analyzed the milk of two ethnically distinct groups of women in northern Nigeria and Nepal. The milk of the Nepalese women (n= 36) contained slightly more BSSL activity (mean, 38.8 units/mL) than that of Fulani (n= 48; mean, 30.3 units/mL) and Kanuri (n= 90; mean 27.6 units/mL) women in Nigeria. There was also a weak positive correlation between the BSSL content of the milk and the body mass index (BMI) of the lactating women. The BSSL activity declined with the length of lactation for both well-nourished and undernourished women. The presence of a heat-stable inhibitor of BSSL in cow's milk was also demonstrated. This finding, along with the decrease in BSSL activity postgestation, could be of significance to populations such as the Fulani of the western Sahel who supplement the diets of their infants with unpasteurized cow's milk.