As compared with other methods, our recently reported method for subcellular fractionation of human platelets improves the separation of mitochondria, alpha granules, and lysosomal enzyme activities. The relative purity of these fractions has led us to undertake the present study to compare the subcellular distribution of phospholipids, fatty acids, and platelet factor 3 (clot-promoting) activity. Two findings pertaining to distribution of phospholipids were entirely new. (1) In the alpha granule zone, plasmalogen phosphatidyl ethanolamine peaked at the expense of diacyl phosphatidyl ethanolamine. (2) The fatty acid composition of the membrane lysophosphatidyl choline suggested that it may have been formed by the action of platelet phospholipase A2 activity. The fatty acids of the membranes showed a markedly asymmetrical distribution in noncholine versus choline phospholipids. The latter held 94%, 72%, and 85%, respectively, of the total content of 16:0, 18:1, and 18:2 fatty acids, whereas 55% of the 18:0, 72% of 20:4, and 67% of higher polyenoic acids other than 20:4 were esterified to the noncholine group. The most important new information related to clot-promoting activity, which, on the basis of protein content, was highest in the membrane fractions, but on the basis of phospholipid content in the nonmembranous fractions. The discussion centers on possible explanations for this novel finding.