HABER et al.1 showed that antibodies elicited in rabbits by copolymers of angiotensin with poly-L-lysine can be used for radioimmunoassay of angiotensin. The following are details and some results of the method we have developed. Comparing the binding affinity of three different rabbit antisera to Val5-angiotensin II amide (‘Hypertensin’, Ciba) and three analogues, we found that these antisera do not quantitatively behave identically. This fact has important consequences for the standardization of immunoassays of angiotensin in human plasma as long as human angiotensin is not available as a chemically pure substance. (Val5-angiotensin has the amino-acid sequence of bovine angiotensin, while Ileu5-angiotensin represents the structure of horse, hog and possibly human angiotensin.)