Platelet levels of magnesium, calcium, sodium and potassium may provide a better index of intracellular cations or total body ion status than serum, erythrocyte or leukocyte cation concentrations. A method is described for the determination of the total magnesium, calcium, sodium and potassium contents of human platelets. The method involves platelet isolation, repeated washing, lysis and measurement of the ions in the lysate. Magnesium and calcium were determined by atomic absorption spectroscopy and sodium and potassium by flame photometry. Platelet total potassium, magnesium, calcium and sodium were 4.5 +/- 1.5, 2.3 +/- 0.6, 1.8 +/- 0.5, 1.5 +/- 0.6 mumol/1 x 10(8) cells, respectively (means +/- SD). The method is simple, repeatable and accurate.