Young piglets weaned onto soya diets frequently develop diarrhoea which may have a dietary and/or immunological component. Piglets abruptly weaned onto soya at 3 weeks of age developed levels of serum IgG anti-soya antibodies almost comparable to those induced by injection with soya protein in adjuvant at 7 weeks. In the piglets primed by feeding, no significant further increase in antibody occurred after subsequent systemic injection. In contrast, secondary responses were observed in age-matched animals, previously primed by injection, and primary responses were obtained in previously naive piglets. The results demonstrate the development of specific unresponsiveness to soya proteins in neonates fed soya, despite the occurrence of an initial vigorous immune response to the fed protein.