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SummaryThe response of Awassi sheep to primary infection with 10000 Haemonchus contortus larvae and subsequent challenge with 20000 larvae was studied. Two types of response to re-infection were observed. The first, characterized by a sharp decline in egg counts, progressive improvement in packed cell volume (PCV) and a marked loss of adult worms and challenge larvae, was considered as self-cure and protection. In the second type of response there was a temporary decline in egg counts, no improvement in PCV and necropsy findings indicated hyperinfection with challenge larvae.