Based on a study of the interaction of the sera from healthy persons (20), patients with coronary disease (20), rheumatic heart disease (20), myocarditis (20), congestive dilated cardiomyopathy (48) it was established that the sera from dilated cardiomyopathy patients exert an inhibitory effect on the contraction of chick embryo myocardial explants in experiments in vitro. Three grades of the inhibitory effect were detected, depending on the patient's status gravity. The sera from healthy persons and patients with the enumerated cardiovascular diseases failed to produce such an effect.