The applicability of selective immunofluorescent staining in conjunction with fluorescent cell-counting for the quantitative assay of virus mixtures is described. Multiple infection of cell monolayers was achieved with simultaneous inoculation by yellow fever, variola, and psittacosis agents in mixture. Each agent was assayed independently and no interference of viral growth occurred. Viral interference, manifested by the exclusion of yellow fever virus, was noted when the psittacosis agent and variola virus were inoculated 20 hours before the former.