Endotoxin-stimulated mouse peritoneal macrophages were found to contain 13-hydroxyoctadecadienoic acid, which was released upon alkaline hydrolysis of the cells. Compared to untreated cells, incubation with LPS increased the content of 13-hydroxyoctadecadienoic acid in macrophage hydrolysates to about 8-fold. Analysis of the material on chiralphase HPLC revealed that it consisted prevalently of 13(S)-hydroxyoctadecadienoic acid. This indicates its enzymatic origine.