We have identified several strains of Escherichia coli which contain immunoglobulin-binding activity on the cell surface. Affinity-purified antibodies ordinarily used as secondary antibodies in immunodetection protocols were bound by 6 of 72 strains of the ECOR reference collection of E. coli. The Fc fragments of both human and sheep immunoglobulin G (IgG) were also bound, demonstrating the nonimmune nature of the phenomenon. Binding of conjugated IgG Fc directly to unfixed cells was observed by fluorescence microscopy. Western blots showed that the immunoglobulin-binding material occurs in the form of multiple bands, with the apparent molecular masses of the most prominent bands exceeding 100 kDa. No two of the strains have the same pattern of bands. The binding activity in extracts was sensitive to proteinase K. The binding activity of intact cells was reduced preferentially by trypsin digestion, demonstrating exposure at the cell surface. Expression of binding activity in Luria-Bertani broth cultures was favored by a temperature of 37 degrees C and entry into stationary phase of growth.