It has been shown that gangliosides exert an influence on serotonin (5-HT) binding and uptake by human platelets. The effect depends on ganglioside structure. In particular, GT1b increases the number of 5-HT binding sites by 40 to 60%, and GM1, GM3 and GD3 decrease it by 20 to 40%. None of the gangliosides tested influences the affinity of the 5-HT binding sites with KD= 60±10 nM. It has been found that loading platelets with GT1b increases the rate of 5-HT uptake and increases the capacity of platelets for this monoamine. It is concluded that gangliosides incorporated into platelet membranes influence the number of 5-HT binding sites on the serotonin transporter.