Groups of South African, farm-reared merino lambs about 4 months old were vaccinated with extracts of adult Haemonchus contortus enriched with H11, an integral membrane protein from the parasite's intestinal microvilli, or with proteins soluble in Tween 20. The lambs were challenged with 5000 infective 3rd-stage larvae. Compared to the adjuvant-injected controls, lambs vaccinated with 350 micrograms/kg liveweight of the H11-enriched extract showed an 89% reduction in parasite egg production and an 88% reduction in total worm burden at post-mortem 35 days post-challenge. The H11-enriched extract contained some protein also present in the Tween 20 extract. Lambs vaccinated with 600 micrograms/kg of protein soluble in Tween 20 showed a 40% reduction in faecal egg counts and 51% reduction in total worm numbers 35 days post-challenge. Animals injected with 20 micrograms/kg of a sub-fraction containing H11 obtained from the H11 extract, showed a 71% reduction in egg output and total worm numbers.