Host responses to a two-worm F. hepatica infection in mice were studied. Mice became seriously debilitated due to the effects of the parasite 18-35 days after infection. Wide-spread necrosis of hepatic tissue was present at this time and was associated with a lymphocytic infiltration throughout the liver, as well as with perivascular lymphocytic infiltrations. Worms migrated to the common bile duct 30-40 days after infection. Liver tissue was repaired rapidly and the majority of the mice recovered. On the basis of the results obtained, the course of a primary infection in mice was separated into three phases: 1) the incubation phase (0-17 days postinfection), 2) the acute phase (18-35 days), and 3) the chronic with repair phase (36-250 days).