This chapter discusses the inhibition of viruses and reviews to the inhibition of pathogenic microorganisms and tumors. Thiosemicarbazones possess chemotherapeutic activity against a wide range of viruses, some malignant tumors, tuberculosis bacteria, several protozoa, and certain fungi. The thiosemicarbazones under consideration are compounds composed of an aromatic ring and a thiosemicarbazide side chain. The structures of the parent molecule, thiosemicarbazide, and of isatin ß-thiosemicarbazone IBT, a typical compound of this group. In more recent studies, John demonstrated a marked in vitro and in vivo inhibitory effect of methyl-IBT on the CVI strain of vaccinia virus, which is the strain currently used for vaccination. Evidence for the drug acting as a chelate compound was obtained by using concentrations of methyl-IBT and concentrations of copper sulfate that by themselves had no effect on either enzyme activity or transforming abilities, but when mixed together demonstrated significant inhibitory activity.