We investigated the capacity of mouse bone marrow-derived macrophages (BMDM) to produce interleukin 1 (IL 1), interleukin-6 (IL 6), and tumor necrosis factor (TNF) upon lipopolysaccharide (LPS) stimulation. BMDM were allowed to differentiate either in the presence of conditioned medium (from WEHI-3 or L cells), or in the presence of recombinant cytokines (IL 3, macrophage-colony stimulating factor [M-CSF], or granulocyte/macrophage-colony stimulating factor [GM-CSF]). Cells were maintained in culture up to 3 weeks and tested at different times. Significant spontaneous cytokine production was never observed. BMDM rapidly acquired the capacity to elaborate cytokine upon LPS activation. LPS-triggered BMDM were able to produce IL 1, IL 6, and TNF, throughout the culture period, although 2- to 3-week-old cells lost their ability to release IL 1 while accumulation of intracellular IL 1 remained unchanged. The dissociation between synthesis and release of IL 1 was not correlated with a significant modification of the specific binding of LPS onto the cell surface.