Young rats cease to absorb antibodies from milk or immune serum in the gut at 20 days of age. Yet antibodies continue to be secreted in the milk of immune mother rats after the young have reached this age. The cessation of absorption by the young is comparatively sudden and shows little age variation. The possibility that it is brought about by extrinsic factors in the food has been investigated. It was found that young rats fostered on the mothers of litters older than themselves continued to absorb antibodies beyond the age at which the foster-mother's own young would have ceased to do so. Conversely, young rats fostered on the mohters of litters younger than themselves ceased to absorb antibodies at the normal age, although the foster-mother's own young would have continued to do so. Young rats prevented from taking solid food and compelled to live entirely on milk beyond the normal age nevertheless ceased to absorb antibodies at 20 days of age. Conversely, young rats fed on solid food before the normal age continued to absorb antibodies until 20 days of age. Hence it does not appear that the food is responsible for bringing about the change in the permeability of the gut of the young rat to antibodies. Suckling rats given a single injection of Salmonella pullorum at from 10 to 23 days of age produced comparatively high titres of circulating antibody after 8 or 10 days.