We have developed a method for assessing the number of infectious viral particles by measuring what we call fluorescence initiating units (FIV). The present work has been done with HIV, but the methods should be applicable to other viruses as well. Briefly described, cells are mixed with virus and then cast in an agar gel to block further transfer of virus. After a period of incubation sufficient to allow infected cells to express virus antigens, the percentage of infected cells is determined by indirect immunofluorescence.