Human milk from 38 mothers was analysed by radioimmunological method for content of bovine beta-lactoglobulin. Detectable amounts (5-33 micrograms/l) of immunoactive beta-lactoglobulin were found in 18 human milk samples. Milk from 3 mothers, whose infants suffered from infantile colic contained high amounts of beta-lactoglobulin (32, 18 and 14 micrograms/l respectively). With the mothers on a cow's milk free diet the contents fell to non-detectable amounts in two mothers and to 6 micrograms/l in the third. All three infants became free from colic.