Using a number of analogs and fragments of a short-chain peptide bradykinin, a series of experiments have been carried out to assess the effect of modifications to the basic structure of the parent molecule on its myotropic and immunoreactive properties. Binding kinetics of both an antibody raised against the authentic nonapeptide and its specific biological receptor found in the guinea pig ileum were used to study these alteration effects. Peptide derivatives of bradykinin with an extension at the N-terminal (Lys- and Met-Lys-bradykinin) cross-react with the antibody raised to bradykinin 59 and 70% respectively. On the other hand, internal fragments with intact C-termini (2-9 and 3-9 bradykinin) react with this same antibody to an extent of 250 and 875% respectively, indicating that they are more potent antigens than the vasopressor molecule itself. Other internal fragments, as well as 9-substituted analogs effectively and not interact. These results indicated that the C terminal arginine of bradykinin is indeed essential in the binding mechanism with its antibody. This in turn illustrates the role of the carrier ovalbumin in the development of antiserum to the ovalbumin-toluene-diisocyanate-bradykinin complex. The physiological experiments with the guinea pig bioassay preparations lead to similar conclusions. Most internal fragments of bradykinin are devoid of activity, whereas N-terminal fragments (2-9, 3-9, and 5-9 bradykinin) have retained some activity again indicating a need for an intact arginine residue at the C-terminus of the molecule. Any modification in position 9 results in severe impairment of biological activity. Thus, the C-terminal residue of bradykinin must be conserved in order that the molecule may retain its immunological and physiological activities. Any extensions, deletions, or modifications of this site will severely retard these functions.