Anticancer agents used for the treatment of hematological malignancies are known to cause long-term toxicities to the structure and function of bone marrow microenvironment. These obbservations are based predominantly on the in vivo experiments with anticancer drug-treated mice and also on the clinical features of the patients who survived after inhtensive chemotherapies. In the present study, in order to evaluate immediate gene response of human marrow stromal cells to anticancer drugs, we tested with real-time PCR for the expression of various cytokine mRNAs from the stromal cells treated with anticancer drugs. Contrary to our expectation, we found that treatment of stromal cells with cytarabine (Ara-C; 0.1–100 μmol) for 3 to 14 days significantly and dose-dependently enhances the expression of mRNAs of stem cell factor (SCF) and leukemia inhibitory factor (LIF) while downregulates interleukin-6 (IL-6) mRNA. On the other hand, carboplatin (0.1–100 μmol) up-regulated only SCF mRNA and adriamycin (0.001–1 μmol) did not affect these gene expressions (n=6). In order to determine the responsive cell elements of stromal cells for these novel responses to Ara-C, mRNAs were independently quantified after separating CD14 positive macrophages and CD45 negative mesenchymal cells from Ara-C-treated stromal cells (n=6). In this additional experiments, we found that the above-mentioned changes in gene expression by Ara-C were provoked predominantly in the stromal macrophage fraction. Based on these observations, we treated the stromal cells established from 6 patients with AML and 11 with non-leukemic subjects with Ara-C for 2 weeks, followed by washing 3-times to eliminate Ara-C. Allogenic CD34 positive cells were then recharged and supportive functions of the stromal cells were evaluated with standard 2-stage long-term cultures. Indeed, the results showed that transient treatment with Ara-C significantly and dose-dependently upregulates supporting function of premature hematopoietic cells in non-leukemic and, although to the less extent, in leukemic stromal cells. These novel short-term stimulatory effects of Ara-C to marrow microenvironment may provide new explanations for some clinical events such as mechanism of rapid recovery of normal hematopoiesis and stem cell mobilization observed several days after completion of intensive chemotherapy to acute leukemia. In addfition, the pattern of gene response of stromal cells induced by Ara-C is of biologically great interest since it is quite different from the already known gene response of human stromal cells provoked by IL-1.