Some highly purified phospholipases A from the venom of viperidae, crotalidae and elapidae were found to hve anticoagulant properties. All phospholipases which exhibited anticoagulant properties are characterized by a high isoelectric point, but not all strongly basic phospholipases are anticoagulant. Anticoagulant phospholipases hydrolyse highly packed monomolecular films of phospholipids without any lag time while non-anticoagulant phospholipases present considerable induction times indicative of a low penetrating power. When the ester linkages in the procoagulant lipids were replaced by the non-hydrolysable ether bonds, the mixture retained its clotting ability even in the presence of phospholipases, thus suggesting that anticoagulant phospholipases prevent clot formation by hydrolysis of phospholipids. This was confirmed by chemical modification of phospholipases, viz. alkylation of the active-centre histidine with 1-bromo-octan-2-one. This modification yielded proteins which had lost their anticoagulant properties but which retained a high affinity for phospholipids.