Growth of several species of bacteria was inhibited by the presence of endodontic gutta-percha cones. Microbiologic analysis, measurement of osmolarity, microscopy, x-ray diffraction analysis, and scanning electron micrography were used to identify the biologically active component that slowly leaches from gutta-percha cones. This component is zinc oxide in the form of small solid particles, from which active, soluble Zn2+ ion is mobilized by hydrolysis. A hypothesis on the "depot" effect of the ZnO particles is formulated, and is used to discuss some earlier reported literature on toxic and antibacterial activity of zinc oxide-containing materials. It is concluded that zinc oxide is not to be considered an inert compound. Its widespread uses in medicine and dentistry seem to reside in its "inert," biocompatible, and astringent properties mainly. The biologically active role of zinc oxide, however, merits further investigation.