Tromexan (3,3[image]-(4-hydroxycoumarinyl)-acetate) in amts. of 0.005 to 0.2 mg. is extracted from 1 ml. of acidified plasma into 5 ml. of 2,2,4-trimethylpentane to the extent of 97.2 [plus or minus] 3.5%. The optical density of such solns. varies in accordance with Beer''s law. On this basis a procedure for estimating plasma and serum concns. of Tromexan by u.-v. spectrophotometry was evolved.