Platelets from normal human volunteers were investigated by ultrastructural morphometry. Preparation of blood samples was done in two different ways in order to evaluate the possible influence of the preparative method on the size and shape of platelets and their organelles. No significant differences could be dmonstrated for any of the measured parameters. Relative volumes of specific granules, mitochondria and the open canalicular system were calculated according to the method of Weibel. Surface and numerical densities of some compartments were determined. Direct measurements of the long and short axes of the platelets were conducted to ascertain further information about their size and shape. Absolute data are in good agreement with the few reports in the literature.