The antiviral activity of thiosemicarbazones against poxviruses was reported by Hamre, Bernstein & Donovick (1950). Thiosemicarbazone derivatives are clinically used in India and Africa. Appearance of drug-resistant and drug-dependent mutants in the course of viral therapy is a potential danger. Appleyard & Way (1966) and Ghendon & Chernos (1972) observed the selection of thiosemicarbazone-derivatives resistant mutants, during growth of the wild-type strain in the presence of these drugs. We shall describe here the isolation and characterization of an isatin-β-thiosemicarbazone (IBT)-dependent mutant (IBTD) of vaccinia virus, which needs IBT for its growth. The characteristics of this mutant are compared with these of the wild-type (wt) strain and an IBT-resistant mutant (IBTR), isolated in our laboratory. The two vaccinia mutants IBTD and IBTR were isolated after treatment of cells infected with wt virus with the mutagenic agent iododeoxyuridine in the presence of IBT.