The effect of prostaglandin E1 in a concentration of 0.1 gamma/ml on the aggregation and disaggregation of thrombocytes and erythrocytes in vitro was studied. Prostaglandin E1 was found to materially reduce the aggregation and to produce disaggregation of thrombocytes. A somewhat diminished aggregation of erythrocytes was also noted. Prostaglandin E1 failed to cause disaggregation of erythrocytes.