The effect of human serum diluted in Tyrode solution (1:1) on the cardiac contractility has been studied. Fragments of the right auricle myocardium from patients with congenital and acquired heart disease have been used to study serum effect on the contraction force. Myocardial strips were repeatedly stimulated with electrical impulses at a frequency of 0.1 to 1.5 Hz. All the perfusion solutions were maintained at 31-33 degrees C and Ca++ ion concentration was 2 +/- 0.2 mM/l. The serum increased the contraction force in the isolated myocardial fibers from patients with congenital and acquired heart disease. Positive inotropic serum action on the myocardium of patients with acquired heart disease was accompanied by a marked diminution of mechanical oscillations. At the same time human serum does not affect the form of force-frequency curves in both types of the preparations. The results suggest that human serum increases calcium ion homeostasis in the cardiac cells of patients during contraction-relaxation cycle.