The production of the inflammatory mediator paf-acether (paf) from human epidermal cells was investigated in vitro. Human epidermal cells, freshly isolated from normal skin or in culture, were incubated in Tyrode's buffer containing 0.25% lipid-free bovine serum albumin in the presence of 2 μM calcium ionophore A23187, at 37°C, for 1 to 60  min. Paf production slightly began at the first min of stimulation, was significant after 10  min, reached a maximum at 20  min (251 ± 25 pg/l × 10⁶ cells, mean ± 1 SD), and decreased thereafter. About 50% of the paf amount produced by epidermal cells was recovered in supernatants. Addition of the non-acetylated pal precursor l-O-octadecyl-sn-glycero-3- phosphocholine, i.e., lyso-paf, at 0.1 μM to epidermal cells during A23187-stimulation did not alter this production. In contrast, addition of acetyl-coenzyme A at 0.1 mM enhanced pal production by 5 times. The material produced by epidermal cells was identical to synthetic paf because: 1) the aggregation of aspirin-treated and ADP-insensitive washed rabbit platelets it induced was inhibited by BN 52021, an antagonist of the paf putative receptor; 2) the factor was inactivated by phospholipase A2 but was insensitive to lipase from Rhizopus arrhizus; 3) it exhibited the same retention time as synthetic paf during standard and reverse-phase (RP) high-pressure liquid chromatography (HPLC) elution. The paf precursors, i.e., lyso-paf and 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine, were also detected in epidermal cells, stimulated with A23187 or not. As determined by RP-HPLC analysis and confirmed by gas chromatography analysis, these precursors and the paf produced by epidermal cells exhibited more than 90% of a hexadecyl chain at the sn-1 position of the molecule. The present results demonstrate the synthesis and release of paf by normal human epidermal cells. Paf production within the epidermis might account for the development of cutaneous inflammation and the pathogenesis of many skin disorders.