1. A method for differentiating, identifying and enumerating lymphocytes from one another among a plurality of cells of hematopoietic origin selected from the group consisting of blood cells, bone marrow cells and lymph node cells which comprises fixing a biopsy specimen of said cells in a fixative and subsequently staining said fixed cells with a staining amount of a polymethine dye to obtain a plurality of stained cells having individual color characteristics which permit the differentiation, identification and enumeration of the lymphocytes from one another and from a plurality of cells; said polymethine dye having the formula: ##STR5## wherein R is a substituent selected from the group consisting of hydrogen, chlorine and CH.sub.3 --O--C.dbd.O.