A panel of polyclonal antisera and monoclonal antibodies (MoAb) raised against recombinant human interleukin 1 alpha (rh IL-1 alpha) and beta (rh IL-1 beta) was used to localize IL-1 pools within epidermal compartments and to characterize the immunoreactive species. Interleukin 1 alpha and beta immunoreactive species were detected by Western blot analysis only when epidermal extracts were obtained in extraction buffers containing dithiothreitol (DTT), sodium dodecyl sulfate (SDS), or 2 mercaptoethanol. Together with the 31-kD (intracellular precursor molecule) and the 17-kD (mature, secreted form) species, most of the antisera and MoAb reacted with a protein of 52-kD that was not found in several internal organs, and from which a 31-kD form could be released upon reelectrophoresis. Interleukin 1 beta immunoreactivity was consistently found by immunohistology at the level of the stratum granulosum, where IL-1 alpha immunoreactivity, although less consistently, also localized. Several monoclonal antibodies to IL-1 beta reacted intensively and specifically with epidermal basal cells. At the electron microscopical level, IL-1 beta immunoreactivity was detected in the upper layers of the stratum granulosum; it appeared to be membrane associated and suggested an exocytosis process similar to that involving lamellar bodies. These observations 1) confirm the presence of IL-1 species in the normal unstimulated human epidermis, 2) show that both IL-1 alpha and beta are detectable herein, 3) identify 52-kD IL-1 alpha and beta immunoreactive bands that appear special to the epidermis, and 4) suggest a link between epidermal IL-1 and the differentiation process of the keratinocyte.