Two experiments were performed to investigate the effects of cessation of feeding milk substitute (weaning) on antibody responses to Keyhole Limpet haemocyanin (KLH) in young calves. In the first experiment, 47 calves were weaned at 42 days of age, while a further 47 calves remained unweaned until 84 days of age. All calves were immunized with KLH at 43 and 85 days of age. Serum IgG1, IgG2 and IgA responses to KLH were increased in the early weaned calves compared with the late weaned calves following both primary and secondary immunizations. In the second experiment, 24 calves were immunized with KLH at 21 days of age. Subsequently, at 35 days of age, eight calves were weaned, eight calves were sham-weaned and eight calves remained unweaned. Sham-weaning involved continuing to feed milk substitute of greatly decreased nutritional value in an attempt to isolate nutritional from behavioural effects of weaning. No effects of treatments on antibody responses were detected. It was concluded that weaning affects antibody responses to antigens given near weaning but not to antigens given 2 weeks before weaning.