A sensitive enzyme-linked immunosorbent assay (ELISA) using a monoclonal antibody (ALM-1) was developed for the determination of leukotoxin (9,10-epoxy-12-octadecenoic acid), which was reported to exist in human burned skin and neutrophils, and was regarded as a toxic and/or defensive substance in living beings. The leukotoxin was conjugated with bovine serum albumin (BSA) by means of the mixed anhydride technique as immunogen, and BALB/c mice were immunized over 6 months. The detection limit of leukotoxin was at least as low as 5 ng in this ELISA. This antibody had a strong specificity to leukotoxin and no cross-reactivity with the other analogs tested, and was usable for an immunohistochemical application. By using the antibody, leukotoxin Was immunohistochemically observed not only in neutrophils, but also in alveolar macrophages in an oxygen-exposured rat lung.