In the present paper we show for the first time monoacylglycerol hydrolase in human platelets. No monoacylglycerol hydrolase activity could be demonstrated in the other blood cells. The monoacylglycerol hydrolase of platelets could not be released from the cells by heparin, thus the enzyme is distinct from the postheparin plasma lipases. The enzyme could be solubilized by a non-ionic detergent, Triton X-100. The solubilized monoacylglycerol hydrolase from platelets was optimally active at pH between 7 and 8 and at ionic strength corresponding to [NaCl] between 0.1 and 0.3 M. The optimal assay temperature was 37 degrees C. The enzyme activity was sensitive to HgCl2 but not to NaF. Accordingly, it was stabilized by 2-mercaptoethanol.