A method for assessing the number of infectious particles in preparations of HIV has been developed. Virus was mixed with cells to allow binding of virus. The cells were then cast in an agar gel to block any further transfer of virus between the cells. After 4 days of incubation the cells initially infected with HIV expressed viral antigens. The percentage of infected cells was then determined by indirect immunofluorescence. The method was developed for HIV, but is presumably suitable for any virus that can replicate in cells not attached to a surface.