Sequential nasopharyngeal secretions were collected from 36 breast fed and 14 bottle fed babies followed from birth to 7 weeks of age. Secretions free of covert blood contamination were obtained from only 12 breast fed and 5 bottle fed infants aged less than 1 week. In secretions from all bottle fed babies negative for blood contamination and from 8 bottle fed babies positive for blood contamination no IgA was detected by enzyme-linked immunosorbent assay. All secretions contained IgG and four contained IgM. In contrast, IgA could be shown in the nasopharyngeal secretions of 6 of 12 breast fed babies. All breast fed babies had detectable concentrations of IgG and in two, IgM was detectable. By 7 weeks of age all babies had detectable nasal IgA, IgG, and IgM and there were no differences between breast fed and bottle fed babies. At this time IgG concentrations were low in both groups, having halved since birth. The origins of nasopharyngeal IgA and IgG in infant secretions and their possible role in protection against respiratory virus infection is discussed.