AbstractIn response to concanavalin A, cytoplasmic calcium movement was observed in human platelets, both in the presence of 1 mM Ca2+or 1 mM EGTA in the medium. Concanavalin A also caused the activation of inositide turnover and the production of inositol phosphates, suggesting that activation of phospholipase C occurs. The mechanism by which concanavalin A stimulates phospholipase C does not depend on GTP‐binding transducers, because it was not inhibited by GDPβS, while experiments performed in the presence of cytochalasin B suggested a role for membrane glycoprotein IIb‐IIIa‐cytoskeleton interaction in this process. Ca2+‐proteases and Na+/H+antiport also seemed to be related to concanavalin A‐induced phospholipase C activation, as suggested by experiments performed in the presence of leupeptin and