Abstract. Lipid bodies, nonmembrane-bound cytoplasmic inclusions, serve as repositories of esterified arachidonate and are increased in cells associated with inflammatory reactions. We have evaluated stimuli and mechanisms responsible for lipid body formation within human polymorphonuclear leukocytes (PMNs). Arachidonic acid and oleic acid stimulated dosedependent formation of lipid bodies over 0.5-1 h. Other C20 and C18 fatty acids were less active and demonstrated rank orders as follows: cis-unsaturated fatty acids were much more active than trans-fatty acids, and activity diminished with decreasing numbers of double bonds. Lipid bodies elicited in vitro with cis-fatty acids were ultrastructurally identical to lipid bodies present in PMNs in vivo. Lipid body induction was not because of fatty acid-elicited oxidants or fatty acid-induced ATP depletion. Cis-fatty acidinduced activation of protein kinase C (PKC) was involved in lipid body formation as evidenced by the capacity of other PKC activators, 1-oleoyl-2-acetylglycerol and two active phorbol esters, phorbol myristate acetate, and phorbol 12,13 dibutyrate, but not an inactive phorbol, to induce lipid body formation. The PKC inhibitor, 1-O-hexadecyl-2-O-methylglycerol, inhibited PMN lipid body formation induced by oleic and arachidonic acids and by 1-oleoyl-2-acetylglycerol and phorbol myristate acetate. Other PKC inhibitors (staurosporine, H-7) also inhibited lipid body formation. Formation of lipid bodies in PMNs is a specific cellular response, stimulated by cis-fatty acids and diglycerides and apparently mediated by PKC, which results in the mobilization and deposition of lipids within discrete, ultrastructuraUy defined cytoplasmic domains. zPID bodies are nonmembrane bound, cytoplasmic inclusions present within neutrophilic (49) and eosinophilic (48) leukocytes, fibroblasts (52), endothelial cells (23), and other cell types (23). The genesis and functions of lipid bodies are largely unknown in most cells. In both neutrophils (polymorphonuclear leukocytes; PMNs) L (49) and eosinophils (48), lipid bodies become more prominent in number and size when these leukocytes are engaged in inflammatory responses. Increased numbers of lipid bodies in human PMNs associated with various infectious, neoplastic, and other inflammatory reactions have been demonstrated both within biopsied tissues and in PMNs from blood and exudative effusions (13, 49). In addition, PMNs from experimentaUy elicited peritoneal exudates in rabbits, but not PMNs collected concurrently from rabbit peripheral blood, contained increased numbers of lipid bodies (36). These in vivo findings with human and rabbit PMNs indicated that 1. Abbreviations used in this paper: BHT, butylated hydroxytoluene; fMLE N-formyl-methionyl-leucyl-phenylalanine; HMG, 1-O-hexadecyl-2-O-methylrac-glyeerol; PDBu, phorbol 12,13 dibutyrate; PDD, 40t-phorbol 12,13 dideeanoate; PKC, protein kinase C; PMA, phorbol 12-myristate 13-acetate; PMNs, polymorphonuclear leukocytes (neutrophils); OAG, 1-oleoyl-2-acetyl-rac-glycer...