The R-phycoerythrin goat anti—mouse IgG2b is prepared from affinity-purified antibodies that react the with the Fc portion of the heavy chain of mouse IgG2b. To minimize crossreactivity, the antibody has been adsorbed against mouse IgM, mouse IgA, pooled human sera, purified human paraproteins and mouse isotypes IgG1, IgG2a and IgG3 prior to labeling.