As an outgrowth of the experience obtained from the chemical modification of proteins, a series of techniques to crosslink different proteins chemically has been developed in the last two decades. The conjugation of enzymes to antibodies, particularly to monoclonal antibodies (MAbs), has allowed the universal application of immunological techniques to histochemistry, cytochemistry, quantitative assays (e.g., enzyme-linked immunosorbent assay [ELISA]), and cytogenetics.