Abstract: An experiment was made to compare the inhibiting action of neriifolin on the potas- sium influx into erythrocytes with those of ouabain and digitoxin in incubated cold-stored human blood and to determine the lethal dose of these three cardiac glycosides to guinea pigs. It was found that the inhibiting action on the potassium influx and the lethal effect exhibited by ouabain are the strongest, those by neriifolin are less strong, and the weakest are those by digitoxin. There seems to be a correlation between the potency sequence of these two actions. The theoretical basis of this correlation and the possibility that the determination of the inhibiting actions of cardiac glycosides on the potassium influx into incubated erythrocytes may be used for evaluating the biological activity of the cardiac glycosides, were discussed.