Immunoreactive measurements of Angiotensin II in plasma, relate to a variety of angiotensin peptides with different biological activities. A method is described to differentiate these individual angiotensin peptides. It involves extraction of the peptides from plasma by reversible adsorption to phenylsilyl silica cartridges, separation by an isocratic, ion pairing high-pressure liquid chromatography technique and measurement of the appropriate fractions by radioimmunoassay. In umbilical venous plasma molar concentrations of the smaller angiotensin fragments were found to range between 16 and 25% of the concentrations of the angiotensin II octapeptide. Because some angiotensin antisera show higher affinity for the smaller peptides than for the octapeptide, concentrations of angiotensin II, measured by radioimmunoassay, may be overestimated by up to 35% unless the various angiotensin peptides are adequately separated.