F. hepatica infection of rats caused a prolonged elevation of serum total IgE reflecting the continued presence of live worms in the host. Infection with 40 metacercariae stimulated higher total IgE levels than infection with 20 metacercariae. The parasite specific IgE response was biphasic, the first peak coinciding with the migratory phase in the liver parenchyma and the second with the establishment of flukes in the bile ducts.