Cross-protection studies of gilts exposed to 4 transmissible gastroenteritis viruses--Ilinois (field strain), Miller-3, Miller low passage (M-LP), and Miller high passage (M-HP) tissue culture-adapted--indicated that only the gilt vaccinated with Illinois strain was protected, along with its newborn pigs, against challenge exposure with field virus. Similar results were obtained when the 4 viruses were incubated in vitro with colostrum from each of the 4 vaccinated gilts and subsequently used to orally inoculate newborn pigs. However, when the colostrums were used to neutralize M-HP virus in cell cultures, the neutralization titers were similar, indicating that a close antigenic relationship existed among the viruses. Neutralization studies in cell cultures, using immunoglobulin (Ig) fractions derived from colostrums of sows exposed to Illinois and M-HP virus, indicated that Illinois virus elicited more neutralizing activity in IgA than in the IgG fraction and that M-HP virus elicited more IgG than IgA antibody activity. In another study, Illinois virus was treated with these Ig-enriched fractions and then inoculated into the lumen of the jejunum of 3-day-old pigs. Anti-Illinois IgA was the only class of antibody which prevented replication of the Illinois virus in the intestine. Similar intraintestinal inoculations were used to test invasiveness of untreated Illinois and M-HP viruses. It was demonstrated that Illinois virus caused marked effect on the intestine: shortening of the villi, intestinal distension, edema, and presence of accumulated intestinal fluid within 60 hours after inoculation. The M-HP virus grew in the intestinal cells without affecting the length of the villi. The degree of invasiveness of Illinois or M-HP virus may account for the difference in the antibody class elicited in the colostrums.