Intestinal absorption of macromolecules, using human alpha-lactalbumin (alpha-LA) as a marker, was studied in breast-fed and formula-fed infants with infantile colic. Serum samples taken at 30 and 60 min after an intake of human milk were analyzed for alpha-LA by a competitive radioimmunoassay technique. Breast-fed infants with infantile colic had significantly higher s-alpha-LA levels compared with age-matched breast-fed control infants 0-1 month of age: median value 926 micrograms alpha-LA/l serum/l human milk/kg bodyweight (n = 11) versus 150 (n = 34); 1-2 months of age: 173 (n = 22) versus 31 (n = 16); 2-3 months of age: 132 (n = 8) versus 11 (n = 16). Similarly, formula-fed colicky infants had significantly higher s-alpha-LA levels than age-matched formula-fed control infants 1-2 months of age: median value 126 (n = 12) versus less than 10 (n = 14); 2-3 months of age: 156 (n = 11) versus less than 10 (n = 10). The increased absorption of the macromolecule human alpha-lactalbumin in infantile colic suggests that the gut mucosa is affected in infants with infantile colic.