A vaccine containing integral membrane glycoproteins from the intestine of Haemonchus contortus was evaluated in three groups of eight 5 months old grazing calves, naturally infected by Haemonchus similis, Haemonchus placei and other gastrointestinal nematodes. Vaccinated calves received 5 or 50μg of the antigen and 1mg of saponin adjuvant, while the controls received adjuvant alone, initially three times, 3 weeks apart and then four more times at 6 weeks intervals. Three weeks after the last immunisation all of the calves were euthanised for worm counts. Immunisation stimulated high titre antibodies against the vaccine antigens, reduced the egg output of Haemonchus spp. by 85% and the numbers of H. placei and H. similis by 63% and 32%, respectively, compared with control calves. It was concluded that vaccination with intestinal membrane glycoproteins from H. contortus could substantially reduce the transmission of H. placei and H. similis, thus providing protective benefit downstream. This appears to be the first known successful demonstration of a vaccine protective for cattle naturally exposed to infection with any gastrointestinal nematode parasite.