Large amounts of calcium and phosphorus have been found by electron-microprobe analysis in the dense bodies of frozen-dried, human platelets. The calcium and phosphorus occurred in a fixed ratio that was similar to that in dicalcium ATP. No other elements were detectable except chlorine. Minute, electron-dense disks were found in the surface membranes of glutaraldehyde-fixed platelets, and are very variable in number in any population of normal platelets. Both dense bodies and membrane granules are absent if platelets are fixed in the absence of calcium, and are also absent from aggregated platelets.

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