Precise and simple assay of purified and crude preparations of human adenoviruses is essential for basic and gene therapy research. Previous bioassays used to quantitate adenoviruses (such as the plaque assay or fluorescent focus assay) are time-consuming and subjective in their interpretation. Here we describe a flow cytometric method that eliminates these disadvantages and provides a quantitative and reliable method of focus-forming unit (FFU) assay.