Nineteen infants suspected of having cow's milk protein-sensitive enteropathy were studied. They all showed failure to thrive, diarrhoea and/or vomiting when fed a diet of cow's milk, and improved when their diet was changed to casein hydrolysate. Jejunal biopsy was done before and 18--23 hours after a milk challenge. Of the 19 infants, 12 presented histological evidence of cow's milk protein intolerance. Eight suffered from vomiting and diarrhoea within 9 days of the milk challenge, but in 4 cases the histological abnormalities were not accompanied by clinical symptoms. In one case a chicken meat intolerance was documented. The histological appearance of the intestinal mucosa after chicken challenge was identical to that observed after milk challenge. In our opinion, repeated intestinal biopsies before and after an acute challenge is the best method to establish the diagnosis not only of cow's milk protein intolerance but also of intolerance to other alimentary proteins.