The criteria that are used at present to diagnose cow's milk protein sensitive enteropathy (CMPSE) are based on an in vivo milk challenge which can be hazardous and life threatening. We have used an organ culture model to determine the usefulness of this technique in establishing the diagnosis of CMPSE on the basis of a single biopsy with in vitro milk challenge. Fourteen infants with diarrhoea clinically suspected to have CMPSE were studied prospectively. On the basis of milk challenge studies seven infants had CMPSE. They had clinical reaction to cow's milk with associated histological changes and depression of alkaline phosphatase levels in the jejunal mucosa. In all seven cases parallel changes in alkaline phosphatase levels were noted in the organ culture specimens of initial biopsy subjected to in vitro challenge. The seven control infants tolerated cow's milk and did not have histological changes. The alkaline phosphatase levels were moderately increased in the jejunal mucosa in five of the seven infants. The alkaline phosphatase levels in the organ culture specimens of initial biopsy were increased after in vitro challenge in all seven infants. This study suggests that organ culture methods may be useful in the vitro diagnosis of CMPSE, and also obviate the need for in vivo oral milk challenges and repeated biopsies.