This work represents a study of aspects of passive immunityand the method of its transmission to the Dewly born foal. All the foals examined were agammaglobulinaemicat birth. Traces of antitoxin, too low to be of any protective value, were present in foals born to hyper immune dams. The foals rapidly acquired passive y globulbin by absorption of colostral proteins from the small intestine. The antibody levels attained were somewhatlower than those of the dams' serum at parturition. The passive y globulin and anti toxin declined steadily after 24 hours and br three weeks at age their levels bad been halved. The duration of demonstreble passive immunity did not extend for more than four months of life. The development of active immunity began within the first month of life. Autogenous y globulin was detected after two weeks in the colostrum deprived foals. In foals which received colostrum, y globulin levels approaohing those for adult horses were detected by about four months of age. The mechanism of absorption of colostral proteins was by uptake into the epithelial cells of the small intestine and transfer via the lacteals to the systemic oirculation. The intestine was apparently non-selective in its absorption of maoromolecules. The efficiency of absorption three hours after birth of labelled PVP, a synthetic polymer of similar molecular size to y globulin, was shown to be 22% of the total dose. There followed a linear decline in efficiency of absorption with age to the lowest recorded levels at 20 hours. By 24 hours of life the intestinal epithelium was no longer permeable to marker antibody. Examination of the colostral proteins after absorption revealed that the high molecular weight component, y globulin, was retained in the foal's circulation, but that the smaller size milk proteins were excreted in the urine. This neonatal proteinuria persisted only during the period of intestinal absorption.