The direct F.A. Method has been used for diffe­rentiation of virulent and avirulent lines of influenza viruses. A positive fluorescence was obtained indi­cating the level of virus in the tissues, sorbing with dried mouse powder helped to reduce non-specific staining. The virulent line showed the greatest num­ber of fluorescent cells with the peak at 10 hours. the non-virulent lines showed a gra...