The postweaning IgE and IgG antiovalbumin antibody responses were greater in rat pups which had received a cows' milk-based supplementary feed than in litter mates which had not. This effect was antigen non-specific and might be similar to that of supplementary feeds in the development of atopy in human infants. The total IgE concentration was similar in the two groups. Supplementary feeds did not increase the low values of IgE antiovalbumin in rats suckled by mothers immunised to ovalbumin.