Six murine monoclonal antifluorescyl antibody clones encompassing a defined range of affinities and containing K light chains with IgG1 or IgG2 heavy chains were examined. As the fluorescence ofthe ligand is quenched >90% when fluorescein is bound by antifluorescyl antibodies, fluorescence quenching was assayed to monitor polypeptide reconstitution and active site formation on mixing of resolved heavy (H) Two experimental approaches have generally characterized efforts to understand the organization and expression of immunoglobulin genes. One approach has involved amino acid sequence analyses of myeloma proteins (1, 2) and, more recently, hybridoma proteins (3). Another approach has been the cloning (4) and hybridization (5) of DNAs coding for these complex group ofproteins. Both approaches have contributed to current views of antibody diversity and genetics.Based on such work, antibody molecules are known to consist of two heavy (H) and two light (L) chains, each containing variable (V) and constant (C) regions. Translation of the L chain is preceded by the rearrangement of germ-line genes such that the C region is brought together with one ofmultiple V regions through a joining segment, J (6). Emerging evidence seems to indicate that a similar mechanism exists for H chains (2), although an additional region (D) located between the V and J segments also appears to contribute to diversity (7). It is believed that the number and variety of V, D, and J segment genes present in the germ line, and their combinatorial joining, generates antibody diversity and binding site specificity. Affinity of the antibody for a certain ligand could understandably, although not necessarily, be controlled by related mechanisms of gene rearrangement. Further diversity and refinement of antigen binding may be produced through somatic point mutation (8) or assortment of minigene sets or both (9).It has been shown that a given V gene can associate with any C gene controlling H chains of different classes (10). Likewise, Studies of H and L chain reassociation have involved either myeloma proteins for which the specific immunogen is unknown or a complex heterogeneous mixture ofspecific antibody molecules. The capacity of H and L chains isolated from monoclonal antibodies of the same specificity and various affinities to reassociate or form an active site has not been critically examined. The experimental approach used here has been to determine the ability of homologous and heterologous H and L chains, derived from monoclonal antifluorescyl hybridomas, not only to reassociate but also to form functional active sites. Both preferential recombination between certain H and L chains and a strict requirement for the homologous chains to form an active molecule have been observed. These results have led to a proposed mechanism that may account for the apparent restrictions involving H and L chain interactions.