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The conformation of two anti-poly(D-alanyl) antibodies was studied by small-angle X-ray scattering before and after interaction with hapten. With both samples a volume contraction could be observed upon interaction with the tetra-D-alanine hapten. The anti-poly@-alanyl) antibodies obtained by immunization with poly@-alanyl) diphtheria toxoid showed a decrease of the volume by 10% and a decrease of the radius of gyration by 7.7% when 90% of the binding sites were occupied by hapten. With anti-poly@-alanyl) human serum albumin antibodies, a smaller decrease of the volume of 3.2% and of the radius of gyration of 1.4% was found, when 62% of the binding sites were occupied. Since the other data determined for the antibodies, such as molecular weight, radii of gyration of the cross-section, and form T he interaction of the determinant group on the antigen with the combining site on the antibody represents a uniquely specific pattern of recognition on a molecular level. Antibodies are capable of inducing profound conformational changes in cross-reactive antigens, as exemplified by the conversion of a small synthetic periodic, not yet helical, polypeptide into an a-helical shape upon interaction with antibodies produced against a related ordered periodic polymer of a higher average molecular weight, which possessed under physiological conditions an a-helical structure (Schechter et id,, 1971 ). Similar observations were reported, concerning an arsanilated synthetic periodic polypeptide (ConwayJacobs er a/., 1970) and a peptide derived from myoglobin (Crumpton and Small, 1967). To the same category belongs the reaction of metmyoglobin with anti-apomyoglobin resulting in the release of heme (Crumpton, 1966), the activation of an enzymatically inactive mutant of 6-galactosidase (Rotman and Celada, 1968), stabilization of mutant catalase by complex formation with antibodies to normal catalase (Feinstein et a/., 1971), and the increase in RNase enzymatic activity upon adding anti-RNase antibodies to a mixture of S-protein and S-peptide, the two moieties obtained from RNase upon the cleavage of a single peptide bond by subtilisin (Cinader et al., 1971).It would be appealing to assume that, similarly to the induction of changes in antigen conformation by antibodies, transconformation might occur also within the antibody molecule as a result of its interaction with an antigen or a hapten.