Plaque assay FFU/ml FFU/ml = Foci Forming Units/ml – is a measurement of the concentration of live virus in a given amount of fluid. This is measured by spreading a known amount of fluid over a layer of cultured cells which are infected by the virus, then counting the number of areas in the culture that look infected. Before with pfu/ml, the cells may have dropped off the plate eventually, but in the case of FFU/ml, cells are producing virus, but they do not disappear as quickly. As shown in panel B, the cells will keep growing until they bump into each other. In other words, they have contact inhibition. Cancerous cells have lost this contact inhibition and grow uncontrollably.