The enzyme elastase (EC 184.108.40.206) has proved to be a convenient and extremely sensitive marker for the quantification of neutrophils in cutaneous infiltrates. Fluorometric assay using the synthetic substrate MeOSuc-Ala-Ala-Pro-Val-N-methylcoumarin permitted the measurement of this enzyme in as few as five cells and was linear up to about 1000 cells per sample. The mean activity of lysates of human blood-derived neutrophils was 0.57 +/- 0.08 pmol of 7-amino-4-methyl-coumarin released per hour per neutrophil. Extracts of normal human skin contained no measurable elastase activity but resulted in a slight inhibition of the neutrophil enzyme (mean 12%). Application to the in vivo situation has been demonstrated by the use of leukotriene B4 as chemotactic agent. A reproducible neutrophil infiltrate was found at a dose of 2 ng, well below the threshold for the appearance of microabscesses.