Apyrase activity was detected in the salivary gland of Cimex lectularius. Kinetic, HPLC chromatography, isoelectric focusing gel electrophoresis and thermal sensitivity assays indicated that the enzymatic activity is the result of a true apyrase and not the result of independent or combined different enzyme activities. The apyrase activity was dependent on calcium and not on magnesium, manganese or zinc, and was inhibited in the presence of EDTA. The pH optimum for both, ATPase and ADPase activities was 8.5. A high specific activity of Cimex salivary apyrase for hydrolysis of both ADP and ATP was found. Cimex salivary gland apyrase activity decreased by 43 percent right after insects fed on blood as compared with insects before feeding.The apparent molecular mass of apyrase activity as determined by HPLC molecular sieving chromatography was 79 600 daltons. Additionally, a peak showing apyrase activity was detected with a molecular mass of 21 000. However, the small molecular mass activity shifted to a high molecular mass activity when the salivary gland homogenate was prepared in more alkaline buffer and frozen, thus suggesting that the high molecular weight form of Cimex apyrase may result from aggregation of active small units.The saliva of Cimex lectularius inhibited platelet aggregation induced by ADP; one salivary gland pair was sufficient to completely inhibit the aggregation of 0.1 ml of citrated platelet rich plasma induced by 5 μM ADP. The inhibitor was shown to be a component different than nitric oxide.Altogether the data suggest that Cimex salivary apyrase can act to prevent an important step on hemostasis, platelet aggregation, and thus play an important role during the insect feeding to overcome the vertebrate host hemostatic defense mechanism.