A substantial number of leukemic blast colonies were formed when conditioned medium of human bladder carcinoma cell line 5637 was added as a stimulator. Recombinant colony-stimulating factor (CSF) also stimulated leukemic blast cell proliferation, leading to colony formation. Furthermore, serum CSF levels in some patients with acute myelogenous leukemia (AML), as detected by sensitive enzyme-linked immunosorbent assay (ELISA), were high. These observations prompted us to study further the expressions of hematopoietic growth factor genes. Granulocyte-macrophage CSF (GM-CSF) mRNA was detected in the leukemic blast cells from about 30% of patients with AML by Northern blot analysis using strict hybridization conditions with or without in vitro blast cell enrichment. These findings suggest that the expression of cytokine genes including the GM-CSF gene reflects in vivo phenomena, although no clear relationship between expression of the genes and serum CSF level has been established. Gene encoding tumor necrosis factor alpha (TNF-alpha), lymphotoxin (LT) and transforming growth factor beta (TGF-beta) were sometimes expressed in some malignant hematological cell lines and also some fresh leukemic cells.