The influence of a plasma fraction of human blood, rich in high density lipoprotein (HDL), was investigated on the "in vitro" formation of PGI2 and TXA2. The addition of 1 mg HDL-cholesterol per ml incubation fluid stimulated significantly the biotransformation of prostaglandin H2 into PGI2 by the microsomal fraction of pig aorta. The TXB2 formation capacity of whole clotted blood was inhibited by administration of HDL in a dose dependent manner. These results suggest that added HDL is able to enhance the ratio PGI2:TXA2. This did not depend on the preparation of HDL either by ultracentrifugation or by precipitation.