of the major advantages of genetic engineering is the ability to produce novel, hybrid antibodies. Hybrid antibodies can be assembled using fragments from different antibodies with the objective of assembling novel combinations of antibody-related effector functions. To efficiently achieve this goal it is necessary to have a precise understanding of the structure-function relationships within the antibody molecule. Secondly, it is possible to produce hybrids of antibodies with non-immunoglobulin proteins thereby achieving unique combination of functional properties. In this case it is necessary to consider both the desired functional properties and the means of assembling the protein components so as to maintain these properties. In all cases it is necessary to have the cloned gene segments, appropriate vectors and expression systems.