The protective efficacy of doramectin against sucking lice was evaluated under natural challenge conditions in Canada. Two studies with a similar experimental design were conducted in sequence. In each study, two groups of louse-free cattle received either doramectin at a dose of 200 micrograms/kg by subcutaneous injection or no treatment and were then mixed with louse-infested animals for the winter housing period. All animals were examined for lice on treatment day and thereafter at weekly intervals for 13 weeks. In the preliminary study, acquisition of infestation in the face of a very mild challenge of Linognathus vituli was delayed by a mean period of 49 days in doramectin-treated animals compared with untreated controls. The difference in the delay between the groups was significant (P < or = 0.04). In the second study, test animals experienced a moderate challenge of a mixed infestation of L. vituli and Solenopotes capillatus. Acquisition of infestation was delayed by a mean period of 25.6 days in doramectin-treated animals compared with controls, the difference between the two groups being highly significant (P < or = 0.0001). These studies confirmed the protective efficacy of doramectin against sucking lice under natural challenge conditions showing that infestation is prevented for a period of about 4 weeks following administration of the drug at its recommended dose.