The Epithelial primary cells harvested from the respiratory tract of young mice were infected with HVJ (Hemagglutinating Virus of Japan, Sendai Virus) and, the progeny virus, trypsin treated or not, were titrated by the Indirect Immunofluorescent Technique. The progeny virus of HVJ obtained from mouse epithelial primary cells presented a considerable titer, demonstrating that the viral particles are activated, being capable of infecting LLCMK-2 cells, in wich, the progeny virus were titrated.