Hematology: To Prevent Clotting

Hematologists have long sought ways to prevent the formation of dangerous and possibly fatal blood clots. First there was heparin, extracted from the livers and lungs of beef cattle. Then came coumarins, made from rotted sweet clover. Now some British researchers believe they have found what they want in the venom of a Malayan pit viper, close kin to American rattlesnakes.

The trouble with heparin and some other anticoagulants is that they not only prevent clotting, but may overshoot the mark and make a patient liable to hemorrhage. Doctors in Malaya, treating victims of pit viper bites, noticed that they...