Interleukin 6 (IL 6) induces differentiation of murine myelomonocytic leukemia (M1) cells into mature macrophages. This process is monitored by the sequential appearance of surface markers, induction of intracellular enzymes, and changes in morphology as the cells progress from blast cells to mature macrophages. Differentiation is also associated with growth arrest and accumulation of the differentiating cells in the G0/G1 phase of the cell cycle. Interferon-beta (IFN-beta) is known to be involved in the growth arrest of M1 cells by inducing 2'5'-oligoadenylate synthetase (2',5'-AS). We therefore analyzed whether IL 6 has the potential to trigger the full differentiation program directly or whether its effect on M1 cells is mediated through IFN-beta or through the activation of genes that are typically induced by IFN-beta. We first tested whether IL 6 could induce IFN-beta mRNA. Using a reverse transcription/polymerase chain reaction procedure, we found that IFN-beta mRNA was induced by IL 6. By Northern analysis we determined that IL 6 also caused a significant increase in 2',5'-AS gene expression. IL 6, however, induced the expression of two mRNA species (1.7 and 2.4 kb), whereas IFN-beta mainly induced the expression of the 1.7-kb species. Enhancement of 2',5'-AS gene expression by IL 6 was observed even when protein synthesis was inhibited by cycloheximide. Furthermore, IL 6-induced growth arrest of M1 cells was not inhibited by anti-IFN-beta antibodies. Thus induction of 2',5'-AS gene expression is a primary response to IL-6 and not secondary to the induction of IFN-beta.