1. Effects of concanavalin A (Con A) and other lectins on 5-hydroxytryptamine (5-HT) uptake by rabbit blood platelets and on their ultrastructure were studied. 2. Uptake of [3H]-5-HT by platelets was decreased by application of Con A, E-PHA (lectin from Phaseolus vulgaris) and lentil-PHA (lectin