1. There are relatively few references in the literature on the transfer of passive immunity to the newly born foal. Transmission of immunity has been established as being post-natal in origin. At birth the serum is apparently agammaglobulinaemic and only trace levels of antibody are detectable. Peak levels of immunity are reached a few hours after ingestion of colostrum.

2. Immune proteins derived from the dam's serum are selectively concentrated in the mammary gland before parturition. In the horse the predominant immuno-globulins of colostrum are IgG and IgG(T).

3. The mechanism of absorption of colostral proteins by the newborn animal is complex and there are considerable species differences. In newly born ungulates the uptake is effected into specialized epithelial cells of the small intestine by pinocytosis. This is followed by transfer of the proteins across the cell, their extrusion into the lamina propria and into the lymphatics before they reach the systemic circulation.

4. The absorption of macromolecules occurs without degradation in neonatal ungulates, is non-selective and can be enhanced by solvent factors in the colostrum. The duration of transfer is influenced by stress and the administration of adrenal corticoids.

5. The period of absorption in ungulates is short (< 48 hr.) compared to that in rats and mice (13–21 days). In the foal the small intestine is permeable to macro-molecules for less than 24 hr. after birth.

6. After birth there is a gradual decline in the concentration of protective antibody to insignificant levels by 5–6 months. The foal shows evidence of active immunity from one to 3 months of age.

7. A transient proteinuria is detectable in newly born animals fed colostrum soon after birth. This proteinuria occurred in two phases, initially there were rising levels of urinary protein followed by sharply declining levels to 36 hr. of life. Feeding high protein colostrum during and after the time of intestinal closure did not prolong the period of proteinuria. The constituents of the proteinuria were almost exclusively low molecular weight proteins of milk of colostral origin, β-lactoglobulin and α-lact-albumin.

8. The importance of neonatal infections in the foal has been well documented but there are few accounts of its relation to passive immunity.

9. A natural immunological incompatibility, haemolytic disease, occurs in the horse. The disease can be diagnosed in the mare in late pregnancy and prevented in the foal by completely withholding the dam's colostrum for 24 hr. of life.