We examined the hypothesis that myeloperoxidase (MPO), a plentiful constituent of neutrophils, might serve as a marker for tissue neutrophil content. To completely extract MPO from either neutrophils or skin, hexadecyltrimethylammonium bromide (HTAB) was used to solubilize the enzyme. With this detergent treatment, 97.8 +/- 0.2% of total recoverable MPO was extracted from neutrophils with a single HTAB treatment; 93.1 +/- 1.0% was solubilized with a single treatment of skin. Neutrophil MPO was directly related to neutrophil number; with the dianisidine-H2O2 assay as few as 10(4) neutrophils could be detected. The background level of MPO within uninflamed tissue was 0.385 +/- 0.018 units per gram of tissue, equivalent to only 7.64 +/- 0.36 X 10(5) neutrophils. In experimental staphylococcal infection, skin specimens contained 34.8 +/- 3.8 units MPO per gram, equivalent to 8.55 +/- 0.93 X 10(7) neutrophils. These studies demonstrate that MPO can be used as a marker for skin neutrophil content: it is recoverable from skin in soluble form, and is directly related to neutrophil number. Further, normal skin possesses a low background of MPO compared to that of inflamed skin.