Foals with combined immunodeficiency (CID), a fatal genetic defect in the production of both B and T lymphocytes, are born without immunoglobulins and are unable to synthesise them. CID foals receiving immunoglobulins via the dam's colostrum may live up to four months of age. Those CID foals with failure of passive transfer (FPT) die at a much earlier age. The occurrence of CID is of value in studying passive transfer of immunoglobulins, as no confusion exists as to when passive transfer ends and active synthesis of immunoglobulins begins. A high correlation has been found between early foal disease and deaths and lack of passive transfer of immunoglobulins, even though many of these foals appear to nurse normally during the first few hours post partum. Evaluation of immunoglobulin levels in 24 hour post suckle samples would prove of value not only in diagnosing CID foals, but in recognising FPT in otherwise normal foals.