Digestion of a Waldenström's IgM by pepsin for 3 hr at 37°C in pH 4.1 buffer resulted in the production of a fragment having an so20 w value of 15S. The molecular weight of the 15S fragment, as determined from its sedimentation and diffusion coefficients, was approximately 700,000, while that for the undigested IgM was 840,000. Smaller quantities of F(ab′)2µ and Fab′µ fragments were also found. Reductive cleavage of the high molecular weight fragment demonstrated the pressence of 7S subunits and a small quantity of lower molecular weight materials. The similarity of the subunits derived from the fragment to those from the undigested IgM was established by sedimentation, diffusion and molecular weight studies. The data indicated the presence of four subunits in the 15S fragment. Immunoelectrophoresis and Ouchterlony agar diffusion analysis failed to reveal differences in the subunits from the fragment and the undigested IgM. Tyrptic peptide mapping studies of the chains isolated from the undigested IgM and the 7S subunit of the fragment after extensive reduction and alkylation demonstrated only minor differences. The results indicate that the initial phases of the digestion of IgM by pepsin involve an attack at a single subunit of the molecule.