Immunoglobulin M synthesized by a human lymphoblastoid cell line, LA173, was found to bind specifically to the protein A-bearing Cowan I strain of Staphylococcus aureus. The (3H)-leucine-labeled, secreted IgM from these LA173 cells also formed precipitin complexes with purified protein A. Soluble complexes formed at high protein A/IgM ratios retained the ability to bind to the bacterial surface. Precipitin complexes also were observed in double diffusion Ouchterlony gels with a line of identity formed between the IgM, protein A, and anti-IgM in adjacent wells. Intracellular IgM species from detergent-lysed LA173 cells were bound to S. aureus. Labeled 19S pentamers, 8S monomers, and HL subunits were eluted from the bacteria and identified by velocity sedimentation and SDS agarose-acrylamide gel electrophoresis. In addition, several intermediates migrating between 8S and 19S were detected and shown to contain authentic H and L chains. Binding of the labeled IgM 19S pentamers to staphylococci was not inhibited by prior treatment of the bacteria with an excess of unlabeled human IgG. However, S. aureus saturated with unlabeled IgG did not bind either labeled IgM monomers or labeled IgG. The interaction of this human IgM with S. aureus exhibited a high degree of specificity with quantitative recovery of secreted 19S IgM. Intracellular IgM species were bound selectively by the bacteria with little if any contamination by other cytoplasmic proteins.