Flow-cytometric determination of fluorescence ratios between differently stained particles is dependent on excitation intensity.

By use of a flow cytometer, the fluorescence of cells stained with hematoporphyrin derivative and the fluorescence of plastic beads stained with different dyes were analysed as a function of the intensity of the exciting laser light. The ratios of the fluorescence values of stained and unstained cells as well as of stained cells and beads were sensitively dependent on excitation intensities. As a consequence of this finding, the normalization of cellular fluorescence by use of reference particles needs to be made on a well-defined and reproduced intensity of the exciting laser light.