Antipeptide sera of defined subspecificities were obtained when angiotensin II was unidirectionally conjugated to its carrier protein. Antisera with predominantly amino terminal specificity were produced when guinea-pigs were immunized with angiotensin II conjugated via its carboxy terminus to thyroglobulin. When angiotensin II was coupled via its amino terminus, carboxy terminal-specific antisera were obtained. The same conjugates failed to provide antisera with the corresponding specificities when rabbits were used instead of guinea-pigs. This work demonstrates that the peptide-protein coupling strategy may affect the specificity of the resulting antiserum.