The purpose of this paper is to review a subject that is of interest to the student of immunology as well as being of interest and practical importance to the practicing veterinarian and to the owner and manager of any type of cow-calf operation. The subject is that of passive immunity transferred from the dam to the calf via colostrum and the problem of an acquired immunodeficiency in the neonatal calf due to an inadequate intake of colostrum or due to ingestion of colostrum after intestinal permeability to intact immunoglobulin has ceased.