The efficacy of a levamisole sustained‐release bolus to prevent parasitic bronchitis in calves in their first grazing season was compared to ivermectin treatment at three, eight and thirtheen weeks after turn out. Contamination of the pasture was established by experimentally infected seeder calves. Twenty calves were split into two groups. Ten calves of one group received a bolus at the start of the experiment. In the other group the calves were treated with ivermectin at 21, 56 and 91 days. Two principal calves from each group were killed during the experiment to study histopathological changes. Pairs of tracer calves were introduced on both pastures at intervals of four weeks throughout the grazing period. The permanent calves were challenged with lungworm larvae at housing and slaughtered four weeks later. Both systems prevented parasitic bronchitis. Larval output was completely reduced in the ivermectin‐treated calves while all bolus‐treated calves excreted larvae at certain times. The highest group average was 4 larvae per gram faeces. Eosinophilia, ELISA‐titres and histopathological changes confirmed the differences in larval uptake. Challenge infection was not succesful in either group and no worms were found at slaughter. Weight gain was significantly different at housing in favour of the ivermectin‐treated calves, but after challenge this was reduced due to a higher weight gain in the bolus‐treated calves. The practical consequences of the results have been discussed.