Abstract: A process is disclosed for making a crystallized formulation for staining nuclear DNA to be used in flow cytometery. The crystallized DNA staining formulation and a kit containing the crystallized DNA staining formulation are provided. The staining formulation contains a mixture of a dye, a ribonuclease and a non-ionic hydrophilic surfactant. The process for preparing the formulation comprises mixing the dye, the enzyme and the surfactant and evaporating the mixture under vacuum to form a crystallized residue. The kit comprises the crystallized DNA staining formulation packed in single dose vials, each vial being sufficient for one analytic determination. Optionally, the kit may contain additional vials comprising a diluent used to reconstitute the crystallized residue at the time of use.