Plasma samples from a group of patients with severe essential hypertension were formerly shown to have a cardiotonic action on the isolated frog heart. This effect is significantly greater than that observed with plasma from normal persons or those with other diseases. The material in plasma responsible for this action is a system of 3 proteins. These proteins have been distinguished by fractionation with sodium sulfate, and are found among the serum globulins. The protein, the concentration of which determines the activity of the whole system, is called component L. When applied to the frog heart it becomes strongly bound. Two other proteins are needed, one precipitated between 0 and 17 per cent sodium sulfate, the other between 22 and 30 per cent sodium sulfate. The 0 to 17 fraction is thought to require calcium for its activity, which is bound to the protein. Calcium can be released by the addition of component L in a reaction which is dependent on temperature, reversal being possible in high calcium concentrations in the cold. The protein system described causes contracture of frog heart muscle in the absence of ionic calcium, in contrast to other well-known contracture-causing agents which require the presence of free calcium for activity.