GM-CSF is constitutively secreted by myeloblasts of some patients with AML. Recently it has been shown that IL-1 and TNF-alpha can induce GM-CSF production in fresh leukemic cells of patients who do not release GM-CSF spontaneously. To further characterize this phenomenon we investigated GM-CSF induction by TNF-alpha in two leukemic cell lines, U 937 and KG-1a. KG-1a constitutively expresses GM-CSF mRNA as demonstrated by Northern blots and PCR analysis. Unstimulated U 937 cells contained no detectable GM-CSF transcripts. After incubation with TNF-alpha, GM-CSF specific m-RNA was found in U 937 cells. Only slight increases of GM-CSF transcripts were noted in KG-1a cells after TNF-alpha treatment. In unstimulated cultures, GMCSF concentrations were below 1 pg/ml. After 3 days of culture detectable levels of GM-CSF were found after stimulation with 1 ng/ml TNF-alpha and reached a mean of 11.9 pg/ml for U 937 and 59.3 pg/ml for KG-1a after incubation in 50 ng/ml TNF-alpha. Therefore mechanisms of GM-CSF expression are regulated differently in each of these cell lines.