The antibody response of pregnant swine to transmissible gastroenteritis (TGE) virus was studied, with special reference to the titers and the immunoglobulin (Ig) class of TGE neutralizing antibodies in colostrum and milk. Animals vaccinated twice intramuscularly or intramammarily with live attenuated TGE virus developed high levels of antibodies in serum and colostrum, but the levels in milk declined markedly within a few days post-farrowing. In contrast, animals naturally or experimentally infected with virulent virus generally developed lower levels of antibodies in serum and colostrum but maintained higher levels in milk, as compared to the vaccinated animals. Gel filtration studies indicated that antibodies in milk from vaccinated animals were primarily of the IgG class, whereas those from the naturally or experimentally infected animals were primarily of the IgA class. The ability of sows to transmit a high degree of passive immunity to their suckling progeny was more closely associated with TGE antibodies of the IgA than the IgG class. Present evidence suggests that high levels of TGE antibodies of the IgA class occur in milk as a result of an infection of the intestinal tract. Probable reasons for this are discussed.