The progression of foot-and-mouth disease in sows and their litters of piglets following exposure to O1 Lausanne virus has been investigated. The majority of piglets died with acute myocarditis but without developing vesicles. Deaths occurred before and also when sows began showing early evidence of vesicle formation. This clinical picture parallels that seen at the beginning of both the Normandy 1979 and Britanny 1981 epidemics in France and points to the necessity of including foot-and-mouth disease in the differential diagnosis of acute death among cloven-hoofed livestock and furthermore the importance of extending clinical inspections to adult animals on premises when such events take place. In investigations in which the susceptibility of fattening pigs to infection with O1 Lausanne virus given by the intramuscular route was examined it was found that the 50 per cent infective dose was log (10)4.8 TCID50. This finding suggests that large amounts of virus are required for successful initiation of disease. This is more likely to come from animals already infected than by inoculation with contaminated material.