SummaryTotal intracellular Ca2+and45Ca2+uptake has been studied in blood platelets from subjects with Down's syndrome and matched controls. In Down's subjects, Ca2+levels (85.5 ± 5.9 nmol/10-9platelets) were significantly lower than controls, 174 ± 10.0 nmol/10-9(p<0.0005). A similar reduction was seen in calcium uptake (Down's platelets, 0.79 ± 0.06 nmol/10-9platelets; controls, 1.17 ± 0.07 nmol/10-9platelets,p< 0.005). The low levels of intracellular Ca2+may be related to decreased granular storage of serotonin, and the decreased Ca2+uptake with impaired transport by intracellular Ca2+-accumulating organelles such as the dense tubular system.