The newborn pig relies on colostrum as its sole source for serum antibody and milk for its intestinal antibody during most of the post natal period. Colostrum and milk are well adapted to perform their very different immune functions--immunoglobulin in colostrum being derived from serum, whereas milk antibodies are locally produced in the mammary gland and mirror the immunoglobulin profile of adult intestinal juice. Intramammary vaccination is far superior to intramuscular vaccination because it produces not only a local but a systemic response. Oral vaccination is similarly effective. Vaccination of one mammary gland results in antibody activity in the secretion of all glands. Irrespective of the route of vaccination, antibody activity is found in all immunoglobulin classes. The main site of immunoglobulin-containing cells is the lamina propria of the intestinal tract, suggesting that the gut is a major site of immunoglobulin formation. In the piglet, immunoglobulin producing cells first appear in the gut at the end of the first week of life and reach a mature profile after a month. During this period the piglet is likely to be capable of responding to orally presented antigens.