Colostrum or early breast milk or both from each of 16 healthy women contained agglutinating antibodies for all normal streptococcal inhabitants of the human oral cavity (S. mutans, S. sanguis, S. mitis, and S. salivarius), including those which colonize the neonatal oral cavity in significant numbers. Agglutination correlated with the amount of immunoglobulin A (IgA) binding to bacterial surfaces as measured by mixed reverse passive antiglobulin hemagglutination. Surprisingly, colostral IgA agglutinated our control organism, Brucella abortus. Low levels of colostral or milk IgM and IgG antibodies also reacted with all of the test bacteria. Absorption studies with an enzyme-linked immunosorbent assay showed that a proportion of antibodies in colostrum and early milk is specific for each of the different oral streptococci. Fractionation on Sepharose 4B indicated that 11S secretory IgA is the predominant form of colostral and milk antibody for all of the test bacteria, including B. abortus. No evidence was found that reactions other than antigen-antibody reactions resulted in binding of colostral immunoglobulins by any of the test bacteria.