In this study, levels of naturally occurring secretory IgA (sIgA) antibodies to bacterial isolates representative of the microflora of the human eye (Staphylococcus epidermidis and a Corynebacterium species) and the oral cavity (Streptococcus mutans serotypes c and d) were assessed in three different human external secretions. Tears and parotid saliva samples collected at the same time from 22 healthy subjects and colostrum from 11 healthy women (1-14 days post partum) were assessed for sIgA anti-bacterial antibody levels by an enzyme-linked immunosorbent assay. Significantly higher levels (P less than 0.001) of IgA antibodies to Staphylococcus epidermidis and the Corynebacterium sp. were found in tears than in parotid saliva. Furthermore, higher levels of sIgA antibodies to Streptococcus mutans serotype d occurred in parotid saliva than in tears of these subjects. Although levels of salivary sIgA antibodies to S. mutans serotype c were lower than those seen to serotype d, they were not significantly different from those in tears. However, absolute sIgA anti-serotype c antibodies per mg IgA were higher in saliva than in tears. When sIgA antibody levels to the four bacterial strains were assessed in colostrum, the proportion of sIgA antibodies per mg IgA were much lower than seen in tears or saliva. These results suggest that natural sIgA antibodies which occur in human external secretions are induced by antigen ingestion and stimulation of the common mucosal immune system. However, the local presence of antigen at a mucosal site induces greater clonal expression and results in higher levels of sIgA antibodies than at mucosal sites not exposed to local antigenic stimulation.