Lymphoblast transformation tests were carried out in the presence of α-lactalbumin and β-lactoglobulin. In patients with cow's milk protein intestinal intolerance in seventeen of forty-five (37·8%) the lymphoblast transformation tests were positive. Sensitization in the first month of life seemed to favour lymphoblast transformation. In control children in only four of forty-three (9·5%) the lymphoblast transformation tests were positive, the difference from intolerant patients being significant 0·01 < P < 0·001. Lymphoblast transformation tests were negative in the seven children with active coeliac disease. Although a negative test does not exclude cow's milk protein intolerance, lymphoblast transformation tests can be considered a useful aid in diagnosis because of its specificity.