AbstractStudies are presented on a fragment of human Waldenström macroglobulins, previously detected with immunological methods and termed “γM piece”. Attachment to the 19 S molecule was by disulfide bonds. For its release reduction and alkylation at pH 8.0 was sufficient and no dissociating agents were required. Its electrophoretic mobility was in the α2‐region. It could easily be purified by a combination of preparative electrophoresis on Pevikon at pH 8.6 and gel filtration on Sephadex G‐200 in Tris‐NaCl buffer, pH 8.0. The molecular weight of the fragment isolated in this manner was determined by gel filtration and found to be 48 000. From a column of Sephadex G‐100 in 1Npropionic acid it was eluted with the H chains. The molecular weight determined by acrylamide gel electrophoresis in the presence of sodium dodecyl‐sulfate was 25 400.On the basis of these findings it appears possible that the fragment is released from the 19 S molecule as a dimer, which dissociates under treatment with SDS to yield two chains. The electrophoretic mobility, the size of a single chain, the presence in dimeric IgA and the amino acid analysis suggested that “γM piece” is identical with the fragment d
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