Abstract: The use of omega-acceptor and omega-donor polypeptides (comprising about two-thirds and one-third of the .beta.-galactosidase molecule amino and carboxyl termini, respectively), prepared by recombinant DNA techniques, DNA synthesis, or chemical polypeptide synthesis techniques, which are capable of interacting to form an active enzyme complex having catalytic activity characteristic of .beta.-galactosidase, is described along with improved methods and novel compositions for enzyme complementation assays for qualitative and quantitative determination of a suspected analyte in a sample.