Drops in serum albumen levels and packed serum volume (p. c. v.) percentages were recorded 96 hours and 19 days after an initial infection of parasite-free calves with 40,000 Haemonchus spp. larvae, and after a similar challenge of infected calves. When an initial infection was removed by treatment with "Neguvon" 2 weeks post patency, a challenge dose of 10,000 larvae given 3 weeks later revealed a low level immunity had been acquired. Parasite free calves surgically implanted with 300 mature Haemonchus spp. worms into the abomasum, acquired no immunity to a subsequent challenge of 10,000 larvae.