The absorption of macromolecules was analysed by measuring serum concentrations of human alpha-lactalbumin after feeding human milk, using a competitive radioimmunoassay. The control group consisted of 78 children fed by cows' milk formula; concentrations of alpha-lactalbumin in their serum were low. The median concentrations in the different age groups varied between 7-13 micrograms alpha-lactalbumin/1 serum/1 human milk/kg body weight. Twenty-eight children with cows' milk allergy were studied before introduction of a diet free of cows' milk. Nineteen had gastrointestinal and nine skin symptoms. High serum alpha-lactalbumin concentrations were found. Only two children had alpha-lactalbumin concentrations below 100 micrograms alpha-lactalbumin/1 serum/1 human milk/kg body weight. Altogether 76 children on a diet free of cows' milk were studied directly after a cows' milk challenge; 26 developed symptoms. They had significantly higher serum alpha-lactalbumin values than the 50 children with no symptoms upon challenge. Eighteen of the 26 children with symptoms had serum alpha-lactalbumin concentrations of more than 100 micrograms/1 serum/1 human milk/kg body weight in contrast with two of the 50 with no symptoms. Total serum IgE antibodies were analysed in 43 children. No correlation was found between concentrations of serum IgE and serum alpha-lactalbumin in the children with cows' milk allergy.