The human cytokines tumor necrosis factor (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF) both promote growth and survival of malignant cells from children with juvenile myelomonocytic leukemia (JMML). It has been postulated that TNF stimulates GM-CSF gene expression in an autocrine manner. We found here that the specific inhibition of TNF gene expression by a catalytic RNA molecule (ribozyme) also downregulated the expression of GM-CSF in JMML cells. GM-CSF protein, GM-CSF-dependent colony formation, and viability of JMML cells were reduced. The observed effect was specific, because synthesis of interleukin-1beta, another cytokine produced by JMML cells, was not affected by the ribozyme treatment. The stimulatory effect of TNF on GM-CSF gene expression in JMML cells probably takes place at the transcription level, because the ribozyme treatment decreased GM-CSF mRNA. No apparent toxicity of the ribozyme was detected in normal bone marrow progenitor cells. Thus, the inhibition of TNF gene expression in JMML cells by ribozymes may be a novel therapeutic approach for this disorder.