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A new class of immunoglobulin, IgD, was identified in normal human serum by immunochemical technics. Antiserums prepared against the unique S.J. myeloma protein facilitated recognition of the related normal protein. IgD was shown to possess type K (I) and type L (II) light chain determinants, similar to those present in other classes of immunoglobulins. IgD does not possess determinants which are specific to IgG, IgA, or IgM. The IgD proteins possess their own specific antigenic determinants. IgD migrates in the fast γ-region on immunoelectrophoresis. The properties on sephadex gel filtration and DEAE cellulose chromatography are described. IgD was found to have a median level of 0.03 mg/ml in 100 normal serums. The range of concentrations found in individual normal serums is much wider, however, than that of other classes of immunoglobulins. IgD, on the average, accounts for less than 1 per cent of the normal serum immunoglobulins.