Immunization of 48 cattle with somatic antigen extracts of the nodular worm, Oesophagostomum radiatum, reduced faecal egg counts (FEC) by 51% and worm burden by 47%. The antibody titre of each animal was correlated to both worm burden (r = -0.40, P < 0.005) and FEC (r = -0.37, P < 0.01). The vaccinated cattle showed wide variation in their response to vaccination with worm burdens after challenge infection varying from 0 to 4060. Of these 48 vaccinated animals, eight developed sterile immunity (worm burden = 0, FEC = 0) whereas a further 10 had no protective response to immunization (worm burden > 1,000, FEC > 100). A comparison of these two sub-groups showed that the group with sterile immunity had a higher antibody titre (P < 0.05). In addition, the group with sterile immunity developed a blood eosinophilia between 21 and 29 days after challenge infection. Vaccinated, unprotected calves and control, unvaccinated calves did not develop eosinophilia.