We evaluated automated analysis for the measurement of immature reticulocyte fraction(IRF) in 222 blood samples. IRF was performed by R-3000 & XE-2100(Sysmex), VEGA RETIC LC141(Horiba), ADVIA 120(Bayer medical), CELL DYN4000(Abbott) or GEN'S(Coulter), and compared with measurements obtained by visual counting. A subfraction of IRF was also determined using different modalities on all analyzers. The values of IRF were stable after blood storage for 48 hours at 4 degrees C and 8 hours at room temperature. A good correlation of IRF among these analyzers was obtained(r = 0.629-0.869), but the normal ranges in the percentage of IRF varied according to analyzers. However, an elevation in the percentage of IRF is the first sign of bone marrow recovery after chemotherapy and the bone marrow transplantation in the majority of leukemic patients. We suggest that IRF counting is useful for the estimation of early phase erythropoiesis in patients who have successful bone marrow engraftment following bone marrow transplantation.