Protein D, a novel surface protein of the bacterial species Haemophilus influenzae with affinity for human IgD, was isolated after solubilization with sonication and Sarcosyl-extraction by a single SDS-PAGE step. From 1 ml of packed bacteria was prepared 0.25 mg of purified protein D. The apparent m.w. of protein D was estimated to 42,000 by SDS-PAGE and gel chromatography. Edman degradation cycles of protein D produced no amino acid phenylthiohydantoin derivatives and the amino-terminal end of the single protein D polypeptide chain is thus probably blocked. Protein D differs from all previously described outer membrane proteins (protein 1 to 6) of H. influenzae. Thus, protein D did not react with antibodies against protein 1 or protein 2 and the latter proteins did not bind IgD. Protein D was found to exhibit unique Ig-binding properties. Thus, in dot blots protein D bound four different human IgD myeloma proteins but not IgG, IgM, IgA, IgE, or some additional proteins. On the IgD molecule, constant parts of the H chains both in the Fab and Fc fragments appear responsible for the interaction with protein D. This novel Ig-binding reagent promises to be of theoretical and practical interest in immunologic and microbiologic research.