AbstractThe incubation of macropages (MΦ) in the presence of lipopolysaccharides (LPS) usually results in the release of a variety of immunoregulatory cytokines such as interleukins (IL), tumour necrosis factor (TNF) and colony stimulating factors (CSF). We recently observed that conditioned media (CM) from LPS‐treated murine MΦ lines probably contain another protein endowed with granulocyte stimulatory activity. This cytokine, which has an apparent MW of about 55 kDa enhances the PMA‐induced luminescence of granulocytes and also stimulates their degranulation as measured by lactoferrin release. In contrast to IL1and IL6this factor is destroyed by brief treatment at pH 2, but is stable for 60 minutes at 65°C. Unlike CSF, its activity is unchanged by reducing agents such as beta‐mercaptoethanol. Furthermore, pretreatment of the MΦ with dexamethasone, in order to reduce the release of IL1and TNF, hardly reduces the effect on granulocyte activation. Finally, treatment with a neutralizing polyclonal anti‐murine TNF antiserum only partly abolishes its activity.These results show that, in addition to the already well‐described cytokines, LPS‐treated murine MΦ lines most probably secrete another granu