This F(ab')2 antibody fragment recognizes the mouse gamma heavy chain in IgG and is coupled to fluorescein (FITC) for applications in immunofluorescence such as IF and FACS when binding to Protein-G/A or to live cells with Fc receptors is avoided. The anti-serum is preadsorbed against human serum proteins and affinity purified using a column coupled with mouse IgG. The purified antibody is digested by pepsin to generate the F(ab')2 fragment and then labeled with FITC by using a modified isothiocyanate protocol.