Human milk samples (n = 232) collected during the whole lactation period from 25 healthy, Swedish mothers were analyzed by radioimmunologic method for content of bovine beta-lactoglobulin. Detectable amounts (5-800 micrograms/l) were found in 93 of 232 milk samples (40%). Six mothers had no detectable beta-lactoglobulin in their breast milk on any occasion. Two mothers had measurable beta-lactoglobulin in all their milk samples. No correlation was found between daily cow's milk intake and concentration of beta-lactoglobulin in the milk samples. Six mothers with allergic symptoms such as asthma, hay-fever, eczema all had detectable amounts of beta-lactoglobulin in their milk. Of 19 mothers without allergy, 13 had detectable amounts. This difference did not show statistical significance. The presence of symptoms in the infant such as diarrhoea, vomiting, colic, exanthema was significantly correlated to high levels of beta-lactoglobulin in the milk. Bovine beta-lactoglobulin was also detected in 7 of 13 serum samples. The two mothers with detectable beta-lactoglobulin in all milk samples had the highest serum values, and their infants suffered from gastro-intestinal symptoms, weight decline and exanthema.