A quantitative assay of infective yellow fever virus particles was developed that is based on the enumeration of cells containing fluorescent viral antigen 24 hours after infection of coverslip McCoy cell cultures. The rapidity and efficiency of virus adsorption onto cell monolayers was markedly enhanced by centrifugation of virus inoculum. By this procedure, 95% of virus was adsorbed within 15 minutes and a proportionality was demonstrated between the number of fluorescent cells and the volume of inoculum. The incubation period of 24 hours was established from observations on the sequential development of viral antigen within cells and fluorescent cell counts. The relationship between virus concentration and cell-infecting units of virus was linear; the distribution of fluorescent cells in cell monolayers was random. The sensitivity of the fluorescent cell-counting assay was comparable to that of the method of intracerebral inoculation of mice. A quantitative linear relationship was demonstrated between the quantity of virus neutralized and dilutions of antiserum.