The mean small intestinal intraepithelial lymphocyte count in seven children with untreated cow's milk protein intolerance (CMPI) on a milk-containing diet was significantly higher than 22 control children also having a milk-containing diet. Ten milk-intolerant children on a milk-free diet had a mean intraepithelial lymphocyte count which was significantly lower than the level in the 22 control children on a milk-containing diet. When these 10 children were challenged with cow's milk they relapsed clinically, and in every case the intraepithelial lymphocyte count rose, although it remained within normal limits. Nineteen children on milk-free diets who had recovered from CMPI had a mean lymphocyte count which was also significantly lower than controls on normal diets, suggesting that when milk is remo-ed from the diet the lymphocyte count is low regardless of whether the child is milk sensitive or not. The reaction of intraepithelial lymphocytes to milk in CMPI is markedly different from their response to gluten in coeliac disease.