The efficacy of ivermectin on females of a laboratory strain of Dermanyssus gallinae was tested. Ivermectin produced a noticeable systemic effect on the mites when inoculated intra-abdominally into infested chickens at rates greater than 0.6 mg kg-1. The concentrations efficacious against D. gallinae (1.8-5.4 mg kg-1) were unfavourably close to those causing toxicity in experimental birds (greater than or equal to 5.4 mg kg-1). Ivermectin administered at 5.4 mg kg-1 was active for 70 h post inoculation.