Human leukocyte elastase, a proteolytic enzyme of neutrophils, can be determined by a highly sensitive enzymatic assay. Bathing in hypertonic salt solutions allowed considerable amounts of human leukocyte elastase to be eluted from psoriatic lesions. Optimal elution was achieved with sodium chloride concentrations of 1 M and higher. Thirty patients with psoriasis of varying degrees of severity released significantly increased amounts of human leukocyte elastase following a 10-min bath in salt water. During daily treatment with salt water baths and UV-B radiation the elutable amounts of elastase decreased dramatically within a few days, reaching normal levels when the skin had cleared. Determination of human leukocyte elastase in salt water eluates of psoriatic skin seems to be useful for quantification of therapeutic effects. It appears conceivable that human leukocyte elastase plays a role in the psoriatic tissue reaction.