Abstract: Proteinaceous binding compositions are prepared employing hybrid DNA techogy, where the variable region polypeptides of immunoglobulins are substantially reproduced to provide relatively small protein molecules having binding specificity and lacking the undesirable aspects of the heavy regions of immunoglobulins. The compositions find a wide range of use, particularly for physiological purposes for diagnosis and therapy. The binding compositions may be modified by labeling with radioisotopes, fluorescers, and toxins for specific applications in diagnosis or therapy.