Interleukin 1 (IL 1) is a polypeptide hormone produced by activated macrophages that affects many different cell types involved in immune and inflammatory responses. The cloning and expression of a murine IL 1 cDNA in Escherichia coli encoding a polypeptide precursor of 270 amino acids has been reported, and expression of the carboxy-terminal 156 amino acids of this precursor in E. coli yields biologically active IL 1. By using the murine IL 1 cDNA as a probe, we have isolated its human homolog from cDNA generated to lipopolysaccharide-stimulated human leukocyte mRNA. Nucleotide sequence analysis of this cDNA predicts a protein of analysis of this cDNA predicts a protein of 271 amino acids (termed IL 1 alpha) which shows congruent to 61% homology to its murine counterpart but only 27% homology to a recently characterized human IL 1 precursor (IL 1 beta). We have expressed the carboxy-terminal 154 amino acids of IL 1 alpha in E. coli, purified this protein to homogeneity, and have compared it with pure recombinant murine IL 1 in several different IL 1 assays based on murine and human cells. Recombinant IL 1 is capable of stimulating T cell and fibroblast proliferation and inducing fibroblast collagenase and prostaglandin production, thus proving that a single molecule has many of the activities previously ascribed to only partially purified IL 1 preparations. Our results indicate that there exists a family of at least two human IL 1 genes (alpha and beta) whose dissimilar protein products have similar biological activities.