Interferon-alpha (IFN) induces the enzyme 2-5 oligoadenylate synthetase (2-5 AS) in cells from patients with hairy cell leukemia and B-cell chronic lymphocytic leukemia and this is associated with a breakdown of certain species of cytokine messenger (m)RNA via the activation of a latent ribonuclease. We have studied the expression of the cytokines interleukin 1-beta (IL-1), interleukin 6 (IL-6), granulocyte-macrophage colony-stimulating factor (GM-CSF), and tumour necrosis factor alpha (TNF) as well as of the ribonuclease activator 2-5 AS in the presence and absence of IFN in acute myeloid leukaemia (AML) blast cells from 26 patients. Before monocyte and T-cell depletion there was no expression of IL-1, IL-6 or GM-CSF, and only three of 13 patients studied expressed TNF mRNA. After cell depletion one or more cytokine was expressed in 31-62% of the 26 patients. Expression of one or more mRNA for IL-1, IL-6, GM-CSF and TNF after 18 h incubation was detected in 16 of 26 patients (63%) and this was particularly so in French-American-British (FAB) subtypes M4 and M5. Eight of nine patients with IL-6 mRNA expression and seven of 10 with IL-1 mRNA expression were in the FAB subtypes M4 and M5. Twenty-two of 26 patients showed induction of 2-5 AS mRNA in response to IFN in vitro. Exposure to IFN resulted in reduction of IL-1 mRNA in nine of 12 cases, of IL-6 mRNA in eight of nine, and GM-CSF mRNA in five of seven cases. TNF mRNA was unaffected by IFN despite 2-5 AS induction in 12 of 13 patients expressing this cytokine. In the presence of exogenous IFN, cells from six of seven patients studied showed inhibition of 3H-thymidine incorporation into DNA. DNA synthesis could also be abrogated in six of seven patients with anti-IL-1 monoclonal antibodies (MoAb) and in two of seven with anti-IL-6 MoAb. This inhibitory effect could be reversed in all patients when anti-IL-1 or anti-IL-6 was given in combination with their corresponding cytokine. These data suggest that IFN may exert a therapeutic effect in a proportion of AML patients by blocking IL-1 and IL-6 mediated growth, consequent on activation of the ribonuclease activator 2-5 AS.