Like other benzo-pyrones, coumarin shows serum protein binding. Under in vitro conditions irrespective of the dose which ranged from 50-3.31 μgm/ml binding remained at ∞40%. After i.v. administration an average of 4.6% of the injected dose remained in the vascular system, of which an average of 37% was bound to serum albumins. In case of i.p. administration an average of 4% of the injected dose entered the vascular system of which an average of 36.5% was in the bound form.