Peritoneal macrophages of IL-4–overexpressing mice exhibit a higher 12/15-LOX activity than cells prepared from control animals. Peritoneal lavage cells were prepared from IL-4–overexpressing mice (n = 5) and from corresponding control animals (n = 5). The lavage fluid of two animals was combined, the cells were washed twice with PBS, plated to Petri-dishes, and the macrophages were allowed to adhere overnight. After removal of nonadherent cells, the macrophages were scraped from the dishes and a cell homogenate was incubated in the presence of 100 μmol/L of arachidonic acid for 15 minutes at 37°C. Lipid extraction and RP-HPLC analysis of the LOX products was performed as described in Material and Methods. A representative RP-HPLC chromatogram is shown. The 12-HETE formed was purified by RP-HPLC and was further analyzed for its enantiomer composition by CP-HPLC (inset).