During a nutritional study of 198 infants, seven became allergic to cow's milk. The seven infants showed acute cutaneous manifestations during cow's milk challenge tests in hospital and six had increased levels of IgE cow's milk-specific antibodies. Neither in the development of the levels of immunoglobulins G, A and M, nor in that of the cow's milk-specific antibodies of these isotypes did these seven patients differ from the remaining infants. Beta-lactoglobulin content and levels of cow's milk-, and beta-lactoglobulin-specific antibodies and of immunoglobulins A, G and M were measured in samples of colostrum and milk from the mothers of the seven infants with cow's milk allergy and from a comparison group (non-atopic mothers of non-atopic infants). The milk of the mothers whose infants became allergic to cow's milk contained less IgA through the lactation: 95% confidence intervals of the groups did not overlap. The difference was most marked in the colostrum. All other measurements were similar in the two groups. This suggests that an infant is more likely to develop cow's milk allergy if the mother's colostrum had a low total IgA content.