Failure in colostral immunoglobulin G (IgG) transfer was found in 9 of 87 Thoroughbred foals. Seven (78%) of these 9 foals acquired infections requiring therapy. Twelve of the foals had partial failure in colostral IgG transfer, and 3 of these had infections requiring therapy. The remaining 66 foals had normal transfer of colostral IgG, and only 2 had detectable infections. The failure of colostral IgG transfer was attributable to nursing problems in only one case. When presuckle postpartum colostrum was collected, 2 of 4 failures of colostral IgG transfer and 4 of 6 partial failures of colostral IgG transfer were explained by low colostral IgG content. Of 11 foals from various other sources and dying of infection before 2 weeks of age, 6 had failure of colostral IgG transfer (less than 200 mg IgG/100 ml serum), and 4 had partial failure (200-400 mg IgG/100 ml serum).