Bifunctional antibodies (bABs) having a double specificity to alpha-endorphin (alpha-END) and horseradish peroxidase (HRP) were produced by hybridoma technology. The antibodies constituted about 28-29% of all immunologically active IgG secreted by hybrid hybridoma (quadroma). The quadroma was isolated by fusion of two murine hybridomas (anti-HRP and anti-alpha-END) with distinct phenotypes: double mutant AMD(R)/NAT(S) and its wild type. To produce the double mutant phenotype, an actinomycin D-resistant (AMD(R)) mouse myeloma was used to initiate one of the parental hybridomas. bABs were purified from quadroma culture medium and ascitic fluids by sequential HRP-sepharose and alpha-END-sepharose affinity chromatography. With radioimmunoassay, the affinity of the individual anti-alpha-END combining sites of bABs was shown to be identical to that of parental monoclonal antibodies. Binding to the second antigen (HRP) did not affect the binding of bABs to alpha-END. bABs proved to be efficient for the determination of endorphins and their precursor proopiomelanocortin in immunohistology and immunoblotting.