Kinetics of the S-mercurized horse oxyhemoglobin coagulation has been turbidimetrically studied in the presence of ligands with different affinity for the mercury(II) atom. Coordination of the mercaptide mercury with ligands results in a marked inhibition of the coagulation process, which supports the previously suggested mechanism according to which the mercaptide mercury affects the protein conformation via its additional coordination with one of the neighboring mercury-binding sites, supposedly carboxylate group of an aspartic acid residue. A new insight into the mechanism of the currently practiced mercury-intoxication therapy is discussed.