Equilibrium dialysis was used to study in vitro binding of warfarin at physiological pH and temperature in bovine serum albumin (BSA) along with the increasing concentration of acetaminophen and there interaction with the protein in presence of arsenic. The free fractions were determined by UV spectrophotometric technique. The binding of warfarin to BSA depended on both drug and arsenic concentrations. Free warfarin concentration increased due to addition of acetaminophen which reduced the binding of warfarin to BSA. Free warfarin concentration also increased accordingly by increasing the concentra-tion of acetaminophen when only the BSA was present. When the binding site was blocked by sufficient amount of arsenic the increment of free concentration of warfarin decreased to a lower extent. This suggests that in the presence of arsenic the warfarin being slowly displaced from its high affinity binding site with increasing acetamino-phen concentration.