A new assay for the determination of specific IgE in serum, using a monoclonal anti-IgE (Stallerzym), was evaluated. Compared with the results of skin prick tests, the Stallerzym was found to be 93% specific and 73% sensitive. In sera from patients selected to be either clearly allergic or not allergic (as defined by the concordant results of case history, skin prick test, and provocation test), Stallerzym proved to be as accurate and reliable as the Phadezym IgE RAST currently available.