Two groups of 16 set-stocked calves were used to evaluate a new strategy for the prevention of parasitic bronchitis and parasitic gastroenteritis. One group was left untreated while the calves in the other were treated with abamectin at 0.2 mg/kg at turnout and again six weeks later. The treatment prevented the output of nematode eggs and lungworm larvae in faeces for at least 70 days. The number of infective larvae subsequently appearing on the pasture was reduced by 90.2 per cent and the infectivity of the pasture (as monitored by tracer calves) by 96.0 to 99.8 per cent in the case of Dictyocaulus viviparus, 88.2 to 99.2 per cent for Ostertagia ostertagi and 69.3 to 98.1 per cent for Cooperia oncophora. Parasitic bronchitis occurred in the control calves and both bronchitis and gastroenteritis in the tracer calves grazing the paddock grazed by the control calves, but no disease occurred either in any of the calves treated with abamectin or in the tracer calves grazing the paddock grazed by these calves.