Interleukin-1 (IL-1), a peptide hormone produced by activated macrophages, possesses the ability to modulate the proliferation, maturation and functional activation of a broad spectrum of cell types^1–7 and may play a major role in the initiation and amplification of immune and inflammatory responses through its action on these diverse cell populations⁸. IL-1 exhibits microheterogeneity in terms of its relative molecular mass (M_r, 13,000–19,000) and charge properties⁸, and although murine IL-1 has been purified^9,10 and some of its basic structure–function relationships have been elucidated⁸, it has proved difficult to prepare sufficient amounts of IL-1 for direct and detailed sequence and structural studies. Here we report the cloning, sequence analysis and expression of murine IL-1 cDNA in Escherichia coli. The IL-1 cDNA codes for a polypeptide precursor of 270 amino acids. Biologically active IL-1 was produced in E. coli by expressing the carboxy-terminal 156 amino acids of the IL-1 precursor.