The electron microprobe shows that the dense bodies of human platelets have a mean P:Ca peak ratio of 1–2. After treatment with dry chloroform/methanol this falls to 0-89. These ratios vary slightly from patient to patient. The use of calcium and phosphorus standards enables these peak ratios to be converted to atomic ratios. The size of the phosphorus peak remaining after lipid extraction was given absolute terms with reference to the known quantities of adenine nucleotides and inorganic pyrophosphate in dense bodies. From the mean P:Ca atomic ratio of 1–76 the quantity of calcium in dense bodies was 0-6 mg/10(11) platelets or 2–97 mg Ca/g dry weight of platelets. This is within the published range for total platelet calcium. If all the phosphorus extracted by lipid solvents were phospholipid there would be 5–65 mg/10(11) platelets, and it would occupy most of the space inside dense bodies. The dense bodies of pig platelets contain both magnesium and calcium in a varying ratio to each other. These results are discussed in relation to control mechanisms that may influence aggregation.

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