Excretory-secretory and somatic antigens of Ostertagia ostertagi fourth stage larvae, emulsified with complete Freund's adjuvant, were intraperitoneally administered to calves on three occasions. Two weeks after the last immunisation all calves were infected with a single dose of 130,000 O. ostertagi third stage larvae. All animals were necropsied 25 days after infection. The immunisation procedure resulted in IgG1 and IgG2 antibodies, however no protective immunity was induced as O. ostertagi worm burdens and worm length were similar in all groups.