New synthetic approaches to fluorinated 3-phenyl-1,2,4-benzotriazines for biological testing have been elaborated. 1-(3,4-Difluorophenyl)-3,5-diphenylformazan (IVa) was synthesized via dinitriding of 3,4-difluoroaniline, followed by azo-addition of the resulting azobenzenediazonium chloride with acetaldehyde phenylhydrazone. 6,7-Difluoro-3-phenyl-1,2,4-benzotriazine (Va) was obtained via intramolecular cyclization of formazan IVa in the presence of BF3/AcOH complex. Monofluoro-substituted 6-R-7-fluoro-3-phenyl-1,2,4-benzotriazine derivatives (Vb-Vq) were prepared by substituting fluorine atom with alkoxides in 3,4-difluoronitrobenzene. Conditions for nucleophilic substitution of the second fluorine atom in benzotriazines V have been established. Fluorinated 3-phenyl-1,2,4-benzotriazines have been tested for antiviral and cytotoxic activity on Vero cell cultures and proved to be active against severe diseases caused by smallpox and some other pathogenic viruses.