Cohn's fraction IV-4 of human plasma protein generates by itself a substance with slight pressor activity when incubted without any additives. This renin-like activity was readily inactivated by alkaline treatment. Furthermore, the generated pressor substance, when purified, was found very similar to [Aspl]-[Ile5]-angiotensin-I in the following characteristics; 1) heat-stability, 2) dializability, 3) inactivation by trypsin, 4) Rf value and 5) electrophoretic mobility at various pHs, 6) marked enhancement in pressor and oxytocic activities after incubation with normal human plasma or rabbit's lung extract, and 7) cross reactivity with [AspI]-[Ile5]-angiotensin-I in radioimmunoassay. It is concluded that renin-like activity in the preparation of fraction IV-4 of human plasma protein must be renin or an extremely similar enzyme.