Endogenous immunoglobulins can cause background if labeled secondary detection antibodies bind to them. To block endogenous immunoglobulins on cells or tissue sections, and thus improve specific signal to background, incubate with an excess (20-40 µg/ml) of unconjugated Fab anti-IgG (H+L) antibody just after blocking with normal serum. Blocking efficiency can be confirmed by eliminating the primary antibody from the protocol and incubating with labeled secondary antibody. It may be necessary to increase the concentration of Fab antibody up to 100 µg/ml to suppress signal from high levels of endogenous IgG. To avoid displacement of the Fab antibody by the labeled secondary antibody, a light cross-linking with glutaraldehyde may be necessary, provided that it does not affect antigenicity of the target proteins. Fab antibodies are not effective for blocking immunoglobulins in Western Blotting or ELISA applications.