To the Editor.— Since Sigell and Fless (214:2035, 1970) mentioned the possible role of ascorbic acid in anticoagulant therapy, letters by Rosenthal (215:1671, 1971), Hume et al (219:1479, 1972), and Smith et al (221:1166, 1972) have related observations on the interaction of ascorbic acid in patients, as well as in dogs and rabbits. I wish to briefly review some relevant animal studies, especially those performed in guinea pigs (which, like man, cannot biosynthesize ascorbic acid). Karl Paul Link, "father" of the coumarin anticoagulants, and his coworkers had recognized early that various factors could affect the observed response to dicumarol in animals 1 ; of these, the role of ascorbic acid was in a sense one of the most mysterious. In the early 1940s, Baumann et al 2 showed that administration of ascorbic acid to rats failed to alter either the extent or the duration of the hypoprothrombinemia produced by dicumarol, but