The constitutive and lipopolysaccharide (LPS)-induced secretion of interleukin 1 (IL 1) by cultured human monocytes and macrophages has been studied. Both freshly obtained monocytes and their culture-derived macrophages were induced by LPS to secrete similar amounts of IL 1. Such induction, however, was accompanied by the secretion of dialyzed inhibitory activity. Constitutive secretion of IL 1 was detected in concentrated supernatants of monocyte cultures. The factor obtained constitutively did not manifest significant inhibitory activity. A method is described for the recovery of IL 1-containing supernatants in serum- and other stimulant-free medium. The biological activities of the constitutively secreted IL 1 were similar to the LPS-induced activities. The constitutive secretion of IL 1 was not equally distributed in the entire monocyte population. We found that a small fraction of loosely adherent monocytes secreted higher amounts of IL 1 than the strongly adherent monocytes. However, the property of higher secretion of IL 1 was not stable and disappeared following monocyte cultivation. Thus, constitutive activity of IL 1 could be recovered either by concentrating the culture supernatants or by enriching a subset of monocytes with higher IL 1 activity.