Passive transfer of mucosal antibody to Streptococcus equi was studied in foals during the first 2 months of life. Immunoglobulin G (IgG) and IgA antibodies were found in sera and nasal secretions of foals shortly after colostrum intake. Titers were highest 2 days after birth; IgG predominated in sera, and IgA predominated in nasal washes. Intragastrically administered 99mTc-labeled IgA was transported from the bloodstream to the nasal mucosa of a newborn foal within a few hours of colostrum intake. Western blot analysis of the specificities of colostral and serum antibodies showed that selective transfer of immunoglobulins of defined specificity did not occur. Antibodies from milk samples taken a month or more into lactation had different specificities than those of colostrum or serum samples. Acid-extracted M protein fragments of S. equi recognized by milk antibodies were the same as those recognized by IgG and IgA from nasopharyngeal mucus of horses recently recovered from strangles. We postulate that passive antibody protection of the foal is derived both by secretion of colostral immunoglobulins onto the nasopharyngeal mucosa and by immunoglobulins ingested in milk that directly coat the upper respiratory and oral mucosa during the first months of life.