SummarySera from two blood type B cats had strong isoagglutinating and isohemolyzing titers against blood type A erythrocytes. In order to determine the class of the immunoglobulins, sera from the cats were pooled, ammonium sulfate precipitated, and gel filtered using sepharose 6B to separate the immunoglobulins by molecular size. The immunoglobulin concentrate separated into two fractions. The initial part of the first fraction was shown in an ELISA to contain IgM and to be devoid of IgG by immunoelectrophoresis and to have agglutinating activity (a titer of 1:4 with a protein concentration of 0.8 mg/ml). Treating this fraction with the reducing agent dithiothreitol (DTT) eliminated agglutinating activity. The latter portion of the second column fraction was shown to contain IgG by immunoelectrophoresis, and to be devoid of IgM by ELISA. Agglutinating activity was also present in the second fraction (a titer of 1:2 with a protein concentration of 1.9 mg/ml); hemagglutinating activity was not decreased by DTT treatment. These studies show that the predominant anti‐A isoagglutinating activity in pooled sera from two blood type B cats is IgM and that some isoagglutinin activity can be associated with Ig