The method of the present invention comprises contacting a suspension of viable lymphocytes with a solution containing a membrane potential sensitive fluorescent dye, The contact between the lymphocyte suspension and the MPS dye is maintained for sufficient time to permit labelling of the lymphocytes by the MPS dye material. The labelled lymphocytes are passed individually through a focused source of excitation energy causing any MPS dye material to fluoresce. The fluorescent emission from each cell is recorded as a pulse and the number of pulses of each intensity are recorded, The distribution of the number of pulses versus the intensity is prepared distribution is bimodal. A first peak includes pulses of low flourescence intensity and a second peak comprises pulses of high fluorescence intensity. A ratio of the total counts of low fluorescence intensity to the total counts of high fluorescence intensity produces a factor which distinguishes lymphocytes derived from bodies afflicted with malignancy from bodi
