The conventional lungworm vaccination schedule, requiring a minimum age of eight weeks, a four-week interval between doses and a further interval of not less than two weeks before turnout, was compared with a modified schedule in which the vaccine was given to calves at six and eight weeks old followed by a turnout immediately after the second dose. The control group consisted of unvaccinated calves, half of which corresponded in age to the conventional and half to the modified vaccination group. All groups were turned out to the same pasture area on May 13 with five calves which were artificially infected with Dictyocaulus viviparus so as to ensure adequate pasture challenge. Challenge caused only mild clinical signs in the vaccinated calves but caused severe husk with two fatalities in the controls. Respiratory rates and faeces larval counts for the vaccinated groups were significantly lower than for the control group and, in the main, not significantly affected by vaccination schedule. The vaccinated groups gained more weight than their respective controls of corresponding age and significantly so for both groups during the period when dictyocaulus infection exerted maximum effect. In this experiment, the modified schedule was as effective as the conventional although delayed challenge precluded a satisfactory test for the effect of turnout immediately after the second dose of vaccine. In a repeat experiment involving apparently a very severe pasture challenge, calves vaccinated at six and eight weeks old showed markedly less resistance than those vaccinated according to the conventional schedule. Probable reasons for the difference in outcome are considered.