Fab fragment antibodies are generated by papain digestion of whole IgG antibodies to remove the entire Fc portion, including the hinge region. These antibodies are monovalent, containing only a single antigen binding site. The molecular weight of Fab fragments is about 50 kDa.-Based on immunoelectrophoresis and/or ELISA, the antibody reacts with the Fc portion of rat IgG heavy chain but not with the Fab portion of rat immunoglobulins. No antibody was detected against rat IgM or against non-immunoglobulin serum proteins. The antibody may cross-react with immunoglobulins from other species.