Labeling antibodies with enzymes, fluorochromes, or biotin provides a signal for visualization or quantitation of the target molecule. Antibody bound to agarose is useful for separating a target antigen from a complex mixture. To avoid excessive background staining and to improve sensitivity, only purified antibodies should be used for staining. At the minimum, the IgG fraction, which contains naturally occurring immunoglobulin as well as specific antibody, should be isolated from the antiserum. This can be done by various methods, usually involving a combination of fractionation and chromatography. Preferably, the antibody should be affinity isolated on a column containing the antigen bound to a solid support. This will eliminate all serum proteins, including immunoglobulins that do not specifically bind to the antigen.