An attenuated TGE coronavirus mutant (Nouzilly strain) relatively resistant to acidity and digestive enzymes was used for immunization of 11 seronegative pregnant sows, mainly by the oral route. Nouzilly strain is able to induce a protective lactogenic immunity in suckling piglets against severe virulent TGE challenge. However the level of neutralizing activity in serum and milk of vaccinated sows at the time of challenge exposure is not correlated with passive protection rate. Separation of the immunoglobulin classes bearing neutralizing activity in serum and milk from vaccinated sows, by ion-exchange chromatography, failed to demonstrate any relationship between antibody level of these fractions and passive protection rate of piglets. Our results suggest that IgA neutralizing antibodies present in milk are not the only parameters to consider to explain the lactogenic immunity induced by the Nouzilly strain given by oral route.