Platelets from two probands homozygous for the Chediak-Higashi syndrome have approximately 10% of the normal number of serotonin-containing dense bodies as visualized electron microscopically in air-dried whole mounts. Since transport of serotonin across the platelet plasma membrane proceeds at a normal rate, and the few dense bodies present appear to store normal amounts of serotonin, the absence of dense bodies may account for the low platelet serotonin values found in these patients.