A novel Ig-binding protein has been isolated from the surface of bacteria belonging to the anaerobic species Peptococcus magnus. To solubilize the protein, peptococci were treated with different proteolytic enzymes (papain, pepsin, and trypsin) or with mutanolysin, a bacteriolytic agent known to digest the cell walls of streptococci. Papain, trypsin, and mutanolysin all solubilized peptides showing affinity for radiolabeled human IgG in Western blot analysis. Compared with papain and trypsin, mutanolysin liberated a more homogeneous material, which also had a higher m.w. This mutanolysin-solubilized protein (Mr 95 kDa) was obtained highly purified by a single isolation step on IgG-Sepharose, and the molecule was found to exhibit unique Ig-binding properties. Thus, in dot blots and in Western blots, human IgG, F(ab')2 and Fab fragments of IgG, and human kappa and lambda L chains all showed affinity for the protein. Moreover, the molecule also bound human IgM and IgA, whereas no binding was recorded for IgG-Fc fragments or IgG H chains. Finally, the protein bound to human polyclonal Ig L chains immobilized on polyacrylamide beads. These different data demonstrate that the isolated peptococcal protein binds Ig through L chain interaction. The name protein L is therefore suggested for this novel Ig-binding bacterial cell wall protein.