Flow cytometry (FCM) analysis of CD45, CD45R, CD71 and CD90 expression on Crj: CD (SD) IGS rat bone marrow cells was done after 5-fluorouracil (5-FU) administration to examine whether these lineage-specific cell surface antigens could be myelotoxic biomarkers. The expression of CD45 (CD45Low and CD45High: differing in expression intensity), CD45R, CD71 and CD90 on bone marrow cells coincided with previous reports. After repeated administration of 5-FU at 50 mg/kg/day for 1-5 days, a time-dependent decrease in cells expressing CD45Low, CD71 and CD90 was observed, whereas a decrease in the CD45High expressing cells was not observed. Furthermore, the decrease was dose-dependent in CD45Low, CD71 and CD90 expressing cells after administration of 5-FU between 2 and 50 mg/kg/day for 4 days. After 4-day repeated dose of 5-FU at 50 mg/kg/day followed by a recovery period, the change in number of CD45Low, CD45R, CD71 and CD90 cells to the bottom and in recovery showed different kinetics. In contrast, the change in number of CD45High cells was minimal, and relatively stable after 5-FU administration. The results suggest that CD45, CD45R and CD90 could each be potential myelotoxic biomarkers for a total proportion of common leukocytes including T-and B-lymphocytes, for a total proportion of B-lymphocytes, and for a total proportion of T-lymphocytes plus immature B-lymphocytes and common progenitor cells, respectively. CD71 could be a single myelotoxic biomarker for erythroid cells. Further study is required for isolation of each of the myelo-lymphocytic lineages. However, the present study showed that FCM analysis could be available to assess the lineage or differentiation stagespecific response, such as the different extent and time-course or the kinetics (the time to reach the bottom and to recover to the normal level) of myelotoxic effect in rat bone marrow.