PREPARATION AND PROPERTIES OF THE PEPTIDE CHAINS OF NORMAL HUMAN 19 S GAMMA-GLOBULIN (IGM).

1. A method is described for preparing pure samples of 19s gamma-globulin (IgM) from normal human serum by using successive steps of dialysis, density-gradient ultracentrifugation, chromatography on DEAE-cellulose, and gel filtration on Sephadex G-200. The yield of IgM (20-25mg./100ml. of serum) was equivalent to about one-quarter of that present in normal serum. 2. Analysis of the separated peptide chains of normal IgM and IgG (7s gamma-globulin) showed considerable differences in the amino acid composition of A chains from the two proteins; their respective B chains, on the other hand, were similar in composition. The carbohydrate of both proteins is confined almost entirely to the A chains; the IgM A chain contains about four times as much carbohydrate as the IgG A chain. 3. These findings support the view that the different classes of human immunoglobulin have B chains that are identical and A chains that are chemically distinct.