Abstract On incubation of resident mouse peritoneal macrophages with arachidonic acid several hydroxyacyl derivatives detectable in cellular supernatants are formed. As main products monohydroxyarachidonic acids (monoHETE's) were identified. In addition, smaller amounts of dihydroxyarachidonic acids (diHETE's) supernatants by reversed phase HPLC, normal phase HPLC in combination with UV-spectroscopy and combined gas-chromatography / masspectrometry revealed the presence of 5-, 8-, 12- and 15 - mono-HETE's, two distinct 5, 12-diHETE's, several 8, 15-diHETE's and 14, 15-diHETE. Among the 5, 12-diHETE's, only small amounts of a compound with the characteristics of LTB, were detected. Under the conditions employed, the cycloxygenase products PGE 2 and PGI 2 (as 6-keto-PGF 1g) were only minor metabolites. In contrast, when macrophage cultures were stimulated with the phagocytic stimulus zymosan, PGI 2, PGE 2 and LTC 4 were found as the major conversion products of arachidonic acid, whereas mono- and diHETE's were not formed in detectable amounts.