Summary
Platelet factor 3 activity has been examined in the high (d,1.120) and low (d,1.090) density platelet membranes obtained by the glycerol-lysis technique, and in the lipid fractions extracted from them. The following observations were made : procoagulant activity, as determined by the thromboplastin generation test, was equal in the two types of membrane and was higher than in the intracellular fractions ; the lipid fraction extracted from the low density membranes had a higher activity than that from the high density membranes; at high concentrations this extracted phospholipid had a mild anticoagulant effect. These results demonstrate functional differences between the two types of platelet membrane and favor a plasma membrane origin for platelet factor 3.
