Aim: Cytochrome P450 (CYP) epoxygenases generate epoxyeicosatrienoic acids (EETs) from arachidonic acid, which are attributed with pro-angiogenic and anti-inflammatory properties. Monocyte-derived macrophages are implicated in both processes but it is not known whether monocytes express CYP epoxygenases or generate EETs. Methods & Results: Microsomes were prepared from human blood-derived monocytes and found to generate 11,12- and 14,15-EET from arachidonic acid (LC-MS/MS). Using a gel separation and MS approach we identified CYP2S1 and confirmed its presence in human monocyte-derived macrophages by Western blotting and RT-qPCR. Although CYP2S1 was detectable under basal conditions, its expression increased upon differentiation towards macrophages. When overexpressed together with the human oxidoreductase in SF9 insect cells CYP2S1 microsomes metabolised fatty acids and retinoic acid to their epoxides. The enzyme metabolised arachidonic acid (11,12- and 14,15-EET), linoleic acid (9,10- and 12,13-EpOME) and eicosapentaenoic acid (14,15- and 17,18-EpETE). Retinoic acid was metabolised mainly to its 18-OH-metabolite and 5,6-epoxide. The NADPH-dependent epoxygenation of docosahexaenoic acid (16,17- and 19,20-EpDPE) was in contrast not significant. The enzyme was sensitive to two CYP inhibitors (liarozole and miconazole) but insensitive to the epoxygenase inhibitor MS-PPOH. Moreover, murine Cyp2s1 was detected in two tissues known to be infiltrated with macrophages i.e. Lewis Lung carcinoma and atherosclerotic plaques. Conclusions: CYP2S1 is an epoxygenase in monocytes that is able to metabolise arachidonic acid and retinoic acid metabolites. We hypothesise that CYP2S1 may be implicated in angiogenic events resulting from macrophage activation like in tumour angiogenesis.