The newborn pig and calf are normally devoid of immunoglobulins at birth and rely on colostrum for serum antibody and milk for intestinal antibody during their early life. Colostrum and milk are well adapted to their different immunological roles. Colostrum immunoglobulin is derived almost entirely from the serum of the dam and after absorption by the neonate is destined here to provide serum antibody. The main immunoglobulin in pig's milk is IgA which predominates also in intestinal juice of the adult. Intestinal IgA is locally formed in the lamina propria of the intestinal tract, whereas milk IgA is locally formed in the mammary gland. It seems likely, however, that IgA production at both sites results from antigenic stimulation of the gut associated lymphoid tissue and the immune systems of these organs are intimately linked. The role of IgA in mucosal defence in the calf appears to be assumed by IgG. Colostrum and milk IgG1 is derived from serum; nonetheless it is likely that the main antigenic stimulus for its formation is obtained from the intestinal tract—the mammry gland immune system is present and its artificial stimulation could prove to be of great practical value.