The introduction of pharmacological inhibitors of renin and angiotensin (ANG) converting enzyme in cardiovascular therapy drew new attention to the role of the renin-angiotensin system in regulating circulatory homeostasis. Accurate measurement of very low concentration of the biologically active peptides ANG II and ANG III sometimes in the presence of large amounts of inactive precursors and metabolites is needed. Active and inactive angiotensins have been measured specifically in attomolar quantities by radioimmunoassay after rapid extraction on bonded-phase silica and subsequent high-performance liquid chromatography. ANG II was found to be generated in cold plasma containing conventional inhibitors; it was concluded that renin, converting enzyme, and angiotensinases must be inhibited if physiological ANG concentrations are to be estimated. Modification of the antigen enhanced the specificity of the ANG II antisera. Monoclonal antibodies to ANG II were produced.