Vaccinia virus antigens, in HeLa cells treated with the antipox virus drug isatin p-thiosemicarbazone (IBT), were analyzed by immunoprecipitation, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The total radioactivity in the immunoprecipitate was decreased to almost 50% in the presence ofthe drug as compared to its absence. An inhibition also occurred with an IBT-dependent mutant (IBP) when growing in the absence of IBT. However, similar levels of radioactivity were observed in the immunoprecipitates from an IBT-resistant mutant (IB1') grown in either the absence or presence ofthe drug. When the antigens within the immunoprecipitates were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, it was shown that the inhibition observed in the wild-type-infected cells in the presence of IBT and in the IBP-infected cells in the absence of the drug, was quantitative, affecting the amounts of the different polypeptides more or less equally.Isatin (-thiosemicarbazone (IBT) is an efficient drug active against pox-viruses, which inhibits late stages during virus growth (21). Viral deoxyribonucleic acid (DNA) and early viral proteins are synthesized in the presence of the drug; however, the viral DNA fails to be coated with proteins and remains sensitive to deoxyribonuclease (4,26). Easterbrook (4), who studied vaccinia virus infection in KB cells in the presence of IBT, did not observe inhibition in viral antigen synthesis, even at the late period after infection, using complement fixation and fluorescent-antibody staining techniques. On the other hand, Appleyard et al. (1) followed rabbit pox infection in HeLa cells by precipitation in agar and found that several late viral antigens were missing in the presence of IBT.The aim of the present study was to analyze the antigens formed in the absence and presence of the drug, using the combined method of immunoprecipitation followed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis. An IBT-resistant mutant (IBTR), recently isolated by us@, served as a control in this study. MATERIALS AND METHODS Cells

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