The development of highly specific antisera to human interleukin 1 (IL-1) has been an elusive goal hampered mainly by the availability of only limited amounts of pure immunogen. To surmount this difficulty, three peptides of the major charged species of IL-1 (pI 6.8) were synthesized and covalently coupled to keyhole limpet hemocyanin (KLH). All three peptide-KLH conjugates raised rabbit heterologous antisera that bound intact pure IL-1 in a dose-dependent and domain-specific manner. Immunoblot analysis of crude concentrated culture supernatants with these antisera showed each of them to be highly specific for mature 18-kDa IL-1. Immunoblot analysis of monocyte lysates revealed a single 33-kDa band consistent with the size of the IL-1 precursor molecule deduced from cloned cDNA. These reagents should prove to be valuable tools in the localization and measurement of IL-1 in cells and fluids and may permit the separate study of individual IL-1 species as well as discrete domains of intact IL-1 molecules.