A complementary DNA (cDNA) probe for the predominant (pI 7) form of human monocyte-derived interleukin-1 (IL1) and a collection of 30 human-mouse somatic cell hybrids were used to assign the IL1 gene to human chromosome 2 by Southern blot analysis of hybrid cell DNA digested with the restriction endonuclease BglII. In situ hybridization to human metaphase chromosomes localized the IL1 gene to the long arm of chromosome 2 at position 2q13-2q21 between two fragile sites.