Two transmissible gastroenteritis (TGE) virus mutants (188-SG and 152-SG) were obtained from a low-passage virus strain (D-52) by 188 and 152 cycles of stomach juice treatment and multiplication in cell culture. Compared to the high-passage Purdue-115 and the original D-52 strains, these mutants were more stable at pH 2.0, more resistant to pepsin and trypsin, and characterized by a small plaque phenotype. In vivo, the two mutants were not found to be virulent for 4-day-old piglets and sows after oral inoculation. To test induction of lactogenic immunity, the 188-SG mutant was administered orally to pregnant sows (6 or 7 weeks before parturition) followed by one intramuscular booster (1 week before parturition). After challenge with virulent TGE virus, piglet mortality 7 days after exposure was reduced (to 22%) as compared to the death rate in piglets from control sows (91%).