Ag-B antigen molecules of about 59,000 daltons were partially purified from papain digests of liver cell membranes of Fischer and ACI rats. The preparations were radio-iodinated and the labeled Ag-B antigen molecules were isolated as specific immune complexes with alloantibodies directed to Ag-B1 or Ag-B4. These specifically purified Ag-B antigen molecules were found to give two fragments of 37,000 and 11,000 daltons on sodium sulfate-acrylamide gel electrophoresis. The two fragments (or very similar ones) were isolated from the radioiodinated partially purified Ag-B antigen preparations by acid dissociation and subsequent gel filtration. The 37,000-dalton fragment retained the same Ag-B alloantigenic specificity as the parental 59,000-dalton Ag-B antigen molecules, whereas the 11,000-dalton fragment did not carry any detectable Ag-B alloantigenic activity. In the reaction with rabbit antisera raised against rat cell membranes, each fragment was shown to be antigenically distinctive.