This letter was stimulated by the article by Tsang et al. on a coagulation Factor XIIa inhibitor from adult Schistosoma mansoni (Am. J. Trop. Med. Hyg., 26: 243–247, 1977). Since there is a limited literature on defining anticoagulants in helminthic infections, I felt it important to bring to the readers' attention a paper by G. G. Spellman and H. L. Nossel (Am. J. Physiol., 220: 922–927, 1971). They demonstrated that an extract from dog hookworm inhibited activated Factor X and platelet aggregation.