Specific antibody against free angiotensin II has been raised in high titre by immunizing rabbits with angiotensin II amide (‘Hypertensin’, Ciba) adsorbed on to micro-particles of carbon. The material was injected either parenterally or directly into lymph nodes and spleen. A radioimmunoassay capable of detecting 30 pg of angiotensin II has been developed, and extended for use in the estimation of angiotensin in circulating plasma. The validity of this application of the assay has been established in experiments where plasma levels of this hormone were studied in relation to the intravenous infusion of Val5-angiotensin II amide in humans. Present results of the investigation of the circulating plasma angiotensin II levels in various physiological and pathological states are presented.