The effect of remantadine on the development of viral toxicosis caused in mice by original and remantadine-resistant strains of influenza virus was studied. A definite reduction of the antitoxic effectiveness of remantadine was demonstrated in passages of toxigenic influenza A virus strain in its presence. Prophylactic administration of remantadine protected mice from the development of toxic pulmonary edema and death caused by inoculation of a massive dose of influenza B virus. Passages of influenza B virus in the presence of remantadine produced a strain causing toxicosis in mice the course of which was not affected by remantadine any longer.