Methods for characterizing and distinguishing reticulocytes and mature red blood cells use reagent compositions which include an organic cationic dye for staining the reticulocytes in the blood sample and a buffer solution for maintaining a pH of about 6 to about 9. The dyes may be the red excitable fluorescent dye Oxazine 750, or the blue excitable fluorescent dyes Acridine Orange or derivatives of Acridine Orange. When a zwitterionic surfactant is included in the reagent composition for isovolumetric sphering of the reticulocytes and erythrocytes, and the reagent composition and whole blood sample mixture is passed through the sensing region of a flow cytometer, the light scattered through at least one angular interval and that fluoresced by each cell is measured, the erythrocytes can be distinguished from reticulocytes and the volume, hemoglobin concentration and the hemoglobin content of each reticulocyte or erythrocyte, and the mean cell volume, mean corpuscular hemoglobin concentration, and mean cell hemoglobin of the reticulocytes and/or erythrocytes are calculated from the measured cell-by-cell volume and hemoglobin concentration.