Carefully washed human platelets contain calcium in an unusual state of chemical combination. Platelet calcium is non-ionic and is not exchangeable with ionic Ca45. On extraction of platelets with lipid solvents, the calcium separates with the phospholipids. The calcium-lipid complex thus obtained does not undergo any exchange with radioactive calcium. Study of other properties of platelets reveals that the clot-promoting activity of platelets (activated by antihemophilic factor and plasma thromboplastin component) also separates into the lipid fraction. Preliminary observations are reported on the nature of platelet lipids as revealed by paper chromatography and paper electrophoresis in solvents of low dielectric constant. Small amounts of proteolipid have been isolated from the lipid extracts. The unique calciumlipid complex is thought to originate in the cell membrane of the platelet.