Interleukin-1 (IL-1) is an important immunoregulatory peptide produced by monocytes and macrophages. Because mononuclear phagocytes are morphologically and functionally heterogeneous, we examined whether they differ in their ability to elaborate IL-1. We used discontinuous Percoll gradients to obtain five density-defined human blood monocyte subpopulations. Unfractionated monocytes and their subsets were compared for their ability to stimulate thymocyte proliferation. Supernatants obtained from the denser monocytes consistently contained more IL-1 activity than did supernatants from the less dense cells. This difference in IL-1 activity was the result of differences in IL-1 elaboration, not the selective production of an inhibitor of IL-1- induced thymocyte proliferation. These data demonstrate that density- defined human monocyte subpopulations differ in their capacity to elaborate IL-1.