The pathogenesis of unirradiated, 3 krad-irradiated and 20 krad-irradiated metacercarial infections was compared in zebu calves studied over a 10-week period. Calves exposed to 1000 unirradiated metacercariae (mc) became hypoalbuminaemic, and showed elevated serum concentrations of liver enzymes, whereas neither of the other groups was significantly affected. At slaughter, a mean of 332 flukes was recovered from the 0 krad group, while only 23% and 12% of this number were recovered from the 3 krad and 20 krad groups, respectively. All the worms recovered from the 20 krad group were stunted, and found in biliary ductules, but a mean of 13% of the flukes recovered from the 3 krad group were large, and dwelling in main bile-ducts. Liver lesions typical of acute fascioliasis were present in the 0 krad group, but lesions in the other groups, and particularly the 20 krad group, were far less severe. Judged on clinico-pathological criteria, a single vaccination of calves with 1000 3 krad-irradiated mc induced partial resistance to a challenge with 1000 normal mc eight weeks later, but the reduction in worm recovery was not statistically significant. There was less evidence of protection when two vaccinating doses of 3 krad mc were given within four weeks, with challenge at week 8, and a single vaccination was ineffective aganist a challenge four weeks later. However, when the irradiation dose was increased to 20 krad, a high level of resistance (69% worm reduction) was induced by a single vaccination, given eight weeks before challenge, and liver pathology was strikingly reduced in the vaccinated animals.