A method for flow cytometry evaluation of a biological material for unassociated non-enveloped viral particles having a non-enveloped viral capsid, the method comprising: preparing a fluorescently-stained fluid sample for flow cytometry evaluation, the fluorescently-stained fluid sample comprising: an aqueous liquid medium; a sample of biological material to be evaluated by flow cytometry for the unassociated non-enveloped viral particles having the non-enveloped viral capsid; at least one fluorescent stain to fluorescently stain the unassociated non-enveloped viral particles to prepare unassociated labeled particles of virus size dispersed in the aqueous liquid medium, wherein each said unassociated labeled particle comprise a said unassociated non-enveloped viral particle stained with the at least one fluorescent stain, and wherein each said fluorescent stain has a fluorescent emission response when the unassociated labeled particle is subjected to a stimulation radiation; after the preparing, subjecting the fluorescently-stained fluid sample to flow cytometry evaluation in a flow cytometer, the flow cytometry evaluation comprising flowing the fluorescently-stained fluid sample through an investigation zone of the flow cytometer and in the investigation zone subjecting the fluorescently-stained fluid sample to the stimulation radiation and detecting for the fluorescent emission response from the investigation zone and counting identified occurrences of the unassociated labeled particles; and the preparing the fluorescently-stained fluid sample comprising fluorescent staining the biological material in a fluid sample composition with a said fluorescent stain to prepare a stained fluid sample composition characterized in that the stained fluid sample composition is at an acidic pH in an acidic pH range of from pH 3.0 to pH 6.5. preparing a fluorescently-stained fluid sample for flow cytometry evaluation, the fluorescently-stained fluid sample comprising: an aqueous liquid medium; a sample of biological material to be evaluated by flow cytometry for the unassociated non-enveloped viral particles having the non-enveloped viral capsid; at least one fluorescent stain to fluorescently stain the unassociated non-enveloped viral particles to prepare unassociated labeled particles of virus size dispersed in the aqueous liquid medium, wherein each said unassociated labeled particle comprise a said unassociated non-enveloped viral particle stained with the at least one fluorescent stain, and wherein each said fluorescent stain has a fluorescent emission response when the unassociated labeled particle is subjected to a stimulation radiation; an aqueous liquid medium; a sample of biological material to be evaluated by flow cytometry for the unassociated non-enveloped viral particles having the non-enveloped viral capsid; at least one fluorescent stain to fluorescently stain the unassociated non-enveloped viral particles to prepare unassociated labeled particles of virus size dispersed in the aqueous liquid medium, wherein each said unassociated labeled particle comprise a said unassociated non-enveloped viral particle stained with the at least one fluorescent stain, and wherein each said fluorescent stain has a fluorescent emission response when the unassociated labeled particle is subjected to a stimulation radiation; after the preparing, subjecting the fluorescently-stained fluid sample to flow cytometry evaluation in a flow cytometer, the flow cytometry evaluation comprising flowing the fluorescently-stained fluid sample through an investigation zone of the flow cytometer and in the investigation zone subjecting the fluorescently-stained fluid sample to the stimulation radiation and detecting for the fluorescent emission response from the investigation zone and counting identified occurrences of the unassociated labeled particles; and the preparing the fluorescently-stained fluid sample comprising fluorescent staining the biological material in a fluid sample composition with a said fluorescent stain to prepare a stained fluid sample composition characterized in that the stained fluid sample composition is at an acidic pH in an acidic pH range of from pH 3.0 to pH 6.5.