Breast milk has a high concentration of secretory immunoglobulin and potentially could serve as a source of passive antibody protection of infants against systemic invasion by Haemophilus influenzae type b. Specific antibody to the capsular polysaccharide of this organism was detected in the colostrum and all subsequent milk samples in 11 of 12 women with a radioactive antigen binding assay. The geometric mean concentrations of antibody were 1.99 microgram/ml in colostrum and 0.18 microgram/ml in breast milk at six weeks and after four and one-half to six months of lactation. Antibody levels in colostrum correlated positively with those in subsequent milk samples; levels after six weeks of lactation correlated highly with those present after four and one-half to six months of lactation. IgA was the predominant immunoglobulin class of anticapsular antibody in the colostrum and milk samples as detected by an enzyme-linked immunosorbent assay.