ACTOMYOSIN of vascular smooth muscle appears to be similar in certain properties to actomyosin of uterine smooth muscle1. These properties include solubility, viscosity (ATP sensitivity), and ATPase activity. Needham2 first observed that uterine actomyosin showed an elevated ATPase activity when the potassium chloride concentration was raised from 0.1 M to 0.5 M. Filo et al.3 also observed this to be true for hog carotid artery actomyosin, and we have observed the potassium chloride-induced elevation of ATPase activity of actomyosin of cow carotid artery1. In extending our investigations of contractile proteins of vascular smooth muscle to human blood vessels, we find that in the properties of solubility and viscosity behaviour (ATP sensitivity) the actomyosin of human umbilical artery is similar to the contractile proteins of other vascular smooth muscles. However, with respect to the potassium chloride induced elevation of the ATPase activity, the actomyosin of the human umbilical artery differs from the contractile proteins of other vascular smooth muscles. This report presents data to show that, unlike the actomyosin ATPase enzymes of those smooth muscles studied to date, that of the human umbilical artery is not stimulated by high concentrations of potassium chloride (0.5–0.6 M).