The duration of the anthelmintic activity of abamectin administered by subcutaneous injection at 200 microg/kg was determined using induced infections of Haemonchus placei, Ostertagia ostertagi, Cooperia spp., and Oesophagostomum radiatum in cattle. Twenty-four Friesian calves, raised under worm-free conditions, were assigned to four groups of six animals using a randomised block design based on bodyweight. One group acted as untreated controls and the other three were treated with abamectin 14, 10 or 7 days before infection. At slaughter, 50-52 days after treatment (36-38 days after infection), efficacy was greater than 98-99% against all the parasite species for the groups challenged 10 and 7 days after treatment, whereas no significant activity could be demonstrated 14 days after treatment.