Free secretory component purified from human milk showed considerable charge heterogeneity in the pH range 4.7-6.5 when analysed by isoelectric focusing in thin layer agarose gels. Unlike rabbit secretory component, allotypic variation could not be identified by comparing samples from different individuals. Treatment with neuraminidase resulted in a basic shift of secretory component charge isomers. The charge heterogeneity appeared to be unrelated to the immunoglobulin binding property of secretory component since all charge isomers bound 125I-labelled IgM. 125I-labelled secretory component bound more strongly to purified IgM compared with polymeric IgA, and predominantly to the IgM-containing region of focused normal human serum. Lectin-secretory component interaction was demonstrated by concanavalin A and wheat germ agglutinin binding in a dot-blot nitrocellulose assay and precipitation with concanavalin A by Ouchterlony immunodiffusion. Despite the relatively high carbohydrate composition of both secretory component and the polymeric immunoglobulins, no evidence for lectin-like binding was obtained by sugar inhibition and sugar desorption studies. Similarly, desialation of secretory component did not prevent secretory component-IgM binding. These observations suggest that the charge heterogeneity and sugar composition of secretory component are unrelated to immunoglobulin binding in vitro.