Mononuclear phagocytes release several factors involved in host defense and inflammation. Of these, interleukin-1 (IL-1) has multiple biological activities which are controlled at different levels including modulation of gene expression, protein synthesis or secretion, and interaction with inhibitors. We have investigated the production of IL-1 alpha and beta as well as the production of a specific IL-1 inhibitor (IL-1 INH) during the in vitro maturation of human monocyte-macrophages. Highly purified monocytes isolated by counterflow centrifugal elutriation were cultured up to six weeks, producing high levels of IL-1 alpha and beta during the first week of culture. Shortly after the first week bioactivity of IL-1 decreased, preceding a decrease of IL-1 immunoreactivity. In contrast, IL-1 inhibitory activity reached a peak during the third week and remained detectable up to six weeks. Granulocyte-monocyte-colony-stimulating factor GM-CSF increased the production of IL-1 INH by approximately 20%, but did not affect IL-1 production. The IL-1 INH, apparent molecular weight approximately 23 kD, blocks the binding of [125I]IL-1 alpha to its receptor. The balance between the production of IL-1 and its antagonist may be important for the regulation of the immune response and chronic inflammation during pathological processes.