A number of studies have demonstrated the feasibility of recombining light and heavy chains from a variety of different sources to yield four chain molecules having a conformation indistinguishable from native molecules by several independent criteria. Thus, although competitive recombination experiments have demonstrated preferential reassociation of autologous chains (1–4), hybridization with heterologous light chains, including light chains derived from Igs of different mammalian species, was also readily accomplished (5). Other experiments have shown that the reconstitution of rabbit half molecules results in random hybridization of γ heavy chains creating heteroligating 7S antibody molecules (6) and hybrid H chain allotype pairings (7). The present investigation grew out of published data supporting the concept that during its antibody-secreting lifetime a single cell may be capable of “switching” the class of its antibody product from IgM to IgG (8, 9). This premise raised the possibility that during the period of “switching over” immunoglobulin “monsters” might be synthesized containing both µ and γ heavy chains.