A trial involving 122 calves of approximately 100 kg bodyweight was undertaken to assess the protection against reinfection with Dictyocaulus viviparus in calves whose primary infection was treated with levamisole. Four matched groups, each of 20 housed calves, were taken and one group vaccinated against parasitic bronchitis. Calves in the remaining groups were infected with third stage D viviparus larvae daily for 42 days and treated with 7 . 5 mg/kg levamisole administered subcutaneously at 14, 14 and 28, or 14, 28 and 42 days. All groups were challenged with 10,000 larvae per call on day 49. Relevant control groups were maintained. Eight calves from each main group were slaughtered four weeks later and survivors retained until 20 weeks from the start of the experiment. Calves treated sequentially with two or three doses of levamisole at 14 day intervals while exposed to incoming infection approximated more closely to the performance of vaccinated cattle and were significantly better protected against challenge than those receiving one dose of levamisole only. Feed conversion was most efficient in vaccinated calves not exposed to the potentially lethal trickle infection. After the last levamisole treatment, vaccinated calves and those treated with three doses of levamisole at 14 day intervals were significantly heavier than calves receiving one dose of levamisole only. Vaccinated calves grew heavier than those receiving triple levamisole treatments until the time of challenge. Thereafter the triple levamisole treatment group improved and eventually grew heavier than the vaccinated animals.