The inhibitory activities of 44 anthraquinones against bacterial collagenase were assayed in vitro. Emodin (1,6,8-trihydroxy-3-methylanthraquinone, 41) was the most potent active inhibitor among them (IC50; 4.0 X 10(-5) M). The type of inhibition by 41 was found to be a mixture type by use of Lineweaver-Burk plots. Since the inhibitory activities of anthraquinones were not affected by the concentration of Ca2+ in the assay medium, it is considered that the inhibitory mechanism is not a chelate effect on Ca2+. In mono-substituted anthraquinones, the compounds which have a substituent (OH or SO3Na) at 2-position show more potent inhibitory activity than the corresponding 1-substituted compound.