A fluorescence polarization instrument, in which a heterogeneous population of biological cells can be characterized according to the fluidity of their plasma membranes, is described. The analyzed cells are first labeled with a fluorescent dye specific to the lipid bilayer of the cell membrane, after which the fluidity of each cell membrane is determined separately by recording the degree of fluorescence polarization. The measurements are automatically controlled, and the fluorescence signal is detected when a cell is passing in front of a microscope objective. The distribution of the cell population as a function of the fluorescence polarization is computed and can be stored or displayed at will.