Disclosed is a fast and simple method for quantification of nucleic acid of biological cells as 2-step protocol. In the first step cells are treated with an acidic solution containing a non-ionic detergent and a fluorescent DNA specific label. In the second step the sample may be neutralized. Determining of the content of nucleic can be performed by fluorescence microscopy. The method may also be used for obtaining information of cell cycle analysis, ploidy determination, measurements of nucleotide incorporation and assays for proliferation, health, stress level, apoptosis, necrosis, or other state of conditions of cells. The invention also relates to a kit of parts comprising an acidic agent, a detergent, a labelling agent and optionally a neutralization agent.