AbstractThe constitutive and lipopolysaccharide (LPS)‐induced secretion of interleukin 1 (IL1) 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 IL1. Such induction, however, was accompanied by the secretion of dialyzed inhibitory activity. Constitutive secretion of IL1 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 IL1‐containing supernatants in serum‐ and other stimulant‐free medium. The biological activities of the constitutively secreted IL1 were similar to the LPS‐induced activities.The constitutive secretion of IL1 was not equally distributed in the entire monocyte population. We found that a small fraction of loosely adherent monocytes secreted higher amounts of IL1 than the strongly adherent monocytes. However, the property of higher secretion of IL1 was not stable and disappeared following monocyte cultivation. Thus, constitutive activity of IL1 could be recovered either by concentrating the culture supernatants or by enriching a subset of monocytes with higher I