Flowcytometric enumeration of reticulocytes using RNA-binding fluorochromes, a method both sensitive and reproducible, is rapidly gaining ground. It performs conspicuously better than the traditional manual method of reticulocyte counting, which is unacceptably inaccurate. The reticulocyte count is a valuable indicator of erythropoietic activity, of essential importance in discerning between the aregeneratory anaemias and anaemias accompanied by an adequate, partial or complete compensatory erythropoietic response. The reticulocyte count can be applied to the monitoring of the response to treatment including both allogeneic and autologous bone marrow transplantation, as well as to the evaluation of the response to recombinant human growth factors. The mean fluorescence intensity of reticulocytes enumerated by the flowcytometric analysis, is a variable possessing an independent clinical value for calculation of a reticulocyte production index and an erythropoiesis index.