Effect of taurine (25 mM) on calcium metabolism in human platelets was studied. Ca, Mg-ATPase activity is significantly increased upon addition of 25 mM of taurine to the incubation medium. The activation is absent or reduced in the presence of trifluoroperazine and beta-alanine and after preincubation of platelets with colchicine and verapamil. The rate of ADP (3.5 X 10(-6) M)-induced platelet aggregation is reduced by half upon taurine addition to platelet-rich plasma. The possible role of taurine in intracellular calcium translocation and the following alteration of platelet activity is suggested.