Keratinocytes produce interleukin-1α (IL-1α) and the epithelial variant of its inhibitor, interleukin-1 receptor antagonist (icIL-1ra). Both IL-1α and icIL-1ra lack a secretory signal peptide; however, some icIL-1ra is found in the supernatants of cultured keratinocytes. The lack of correlation with the release of the cytosolic enzyme lactate dehydrogenase suggests that icIL-1ra can be actively secreted. Brefeldin A fails to block icIL-1ra release, suggesting that this protein may be externalized by keratinocytes through a leaderless pathway of secretion. Only minute amounts of soluble extracellular IL-1α are detected: however, both IL-1α and icIL-1ra can be released from the external face of the keratinocyte plasma membrane by mild acidic treatment, suggesting that IL-1α can also be secreted by keratinocytes. The observation of membrane-associated IL-1α and icIL-1ra might reflect an autocrine loop of regulation. Support for this hypothesis comes from the finding that keratinocytes, when exposed to exogenous recombinant IL-1α, increase their content in both IL-1α and IL-1ra mRNA. When keratinocytes are subjected to counterflow centrifugal elutriation, three major cell populations are obtained, representing three different degrees of keratinocyte differentiation. Cells from all populations synthesize IL-1α and IL-1ra: however, while IL-1α is uniformly distributed in cells from all maturational stages, IL-1ra accumulates in large, more differentiated keratinocytes. Changes in the ratio of IL-1ra to IL-1α production and secretion by keratinocytes at different degrees of maturation might contribute to the control of growth and differentiation of human skin.