1.Lipocortin-1 and its N-terminal derivatives exert potent inhibitory actions in various models of acute inflammation. The present study examined the ability of lipocortin (LC)-1 to suppress the release of the acute pro-inflammatory mediators, tumour necrosis factor (TNF alpha) and prostaglandin E2 (PGE2) from human peripheral blood mononuclear cells (PBMC) stimulated with lipopolysaccharide (LPS) or recombinant human interleukin-1 beta (rhIL-1 beta). 2. LPS (10 micrograms ml-1) stimulated release of TNF alpha and PGE2 from PBMC was significantly inhibited by (4 h) co-incubation of the cells with 10(-6) M dexamethasone (Dex), but not with 10(-9) M to 10(-7) M of a N-terminal fragment (amino acids 1-188) of recombinant human LC-1 (LC-1 fragment). However, Dex suppression of LPS-stimulated TNF alpha and PGE2 secretion from PBMC was reversed when polyclonal antibody to LC-1 fragment (1:10,000 dilution) was included in the medium. rhIL-1 beta (5 x 10(-8) M)-stimulated release of TNF alpha and PGE2 from PBMC (after 18 h) was abolished by co-incubation of the cells with 10(-7) M LC-1 fragment. 3. After incubation with Dex (4 h), cellular proteins from PBMC were immunoblotted using anti-LC-1 fragment antibody (which showed to cross-reactivity with human annexins 2 to 6). Dex caused no increase in immunoreactive (ir)LC-1 content of PBMC, although there was a three fold increase in the amount of a lower mass species with LC-1-like immunoreactivity. This was accompanied by the appearance of irLC-1 in the extracellular medium. 4. The results of the present study implicate endogenous LC-1 in glucocorticoid suppression of TNF alpha and PGE2 release from human PBMC and suggest an extracellular site of action for LC-1. LC-1 may also inhibit rhIL-1 beta-stimulated TNF alpha and PGE2 secretion from PBMC.