The IgG1 which is transferred to the udders by an active process of maternal serum at the close of the dry period, constitutes by far the largest fraction in the colostrum. The majority of investigators believe IgA and IgM (and IgG 1 to a less extent) to be mainly synthesised in the udders; the literature is not unanimous as to the extent to which there also is a selective transport of serum to the udders. Increasing attention is particularly being paid in recent years to nutritional immunity which does not only stimulate the immunological response but also affords protection prior to the appearance of the immune response. In order to reduce the period of agammaglobulinaemia as far as possible, early administration of colostrum is essential, the more so as the concentration of Ig in colostrum is highest immediately post-partum.