Twenty-six Friesian steers infected with between 5000 and 45 000 cercariae of Schistosoma mattheei were slaughtered at intervals between 7 and 107 weeks after infection. Lesions developed in the liver, lung and intestine around dead parasites lodging in blood vessels, and many progressed to form lymphoid nodules. The large veins of the intestine became involved in an inflammatory reaction which may be an immunological response to antigen released from dead worms in the veins. The medium-sized intrahepatic portal veins became involved in a reaction which could represent a response to antigen released from dead and dying worms in the mesenteric veins. In heavy infections liver and lungs became grey with pigment regurgitated by the parasites.