Purpose: To quantify virus-like particles in culture or environmental samples.Summary: Host cells and virus-like particles are enumerated from the same glutaraldehyde-fixed sample. For virus counts, an aliquot is diluted in 0.02-µm-filtered TE buffer prior to staining with SYBR Green I nucleic acid dye to improve staining efficiency and ensure an appropriately low event rate in the instrument. An unstained aliquot of sample is run for host cell counts.