Monovalent Fab fragments of affinity-purified, secondary antibodies are offered to cover (block) the surface of immunoglobulins for double labeling primary antibodies from the same host species, or to block endogenous immunoglobulins in tissue sections or on cell surfaces. They can be used for these purposes because each Fab fragment has only a single antigen binding site. HUMAN IgM (myeloma) Fab µ fragment can be utilized as a control or standard reagent in Western Blotting and ELISA experiments. Specific conditions should be optimized by the user.