When synthesis of vaccinia DNA was inhibited, no haemagglutinin (HA) was produced. The antiviral compound isatin-β-thiosemicarbazone (IBT) also arrested the formation of vaccinia HA. It is therefore concluded that the synthesis of vaccinia HA depends on late protein synthesis. We have confirmed earlier findings that 5-bromodeoxyuridine (BUdR) does not inhibit HA production in doses which completely arrest production of infectious virus. In contrast to earlier findings, mitomycin C inhibited HA production in our experiments. The new antiviral compound rifampicin had no or little effect upon vaccinia HA production.