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Abstract: The protective capacity of an adult stage Ostertagia ostertagi globin antigen was tested in four vaccination experiments in cattle. In a preliminary experiment, calves were vaccinated three times intraperitoneally with 250μg globin in Freund''s adjuvant and challenged with a trickled infection of 25,000 infective larvae. In three subsequent field studies, calves were vaccinated twice or three times intramuscularly with 80–100μg globin in Quil A and challenged with a natural gastrointestinal nematode infection on pasture. Higher globin-specific antibody levels were detected in the vaccinated calves than in the control animals in all vaccine trials. In the preliminary experiment, geometric mean cumulative egg counts in the globin group were reduced by 52% and total worm burdens were reduced by 28%, compared to the controls. In the first field trial cumulative faecal egg counts were reduced by 63% in the vaccinated calves. However, the reduction in faecal egg output in these two experiments was not statistically significant and no reduction in faecal egg counts was observed in the vaccinated animals in the two last field trials. In conclusion, vaccination of calves with O. ostertagi globin resulted in highly variable protection levels after challenge infection. [Copyright &y& Elsevier]