Adenosine deaminase, which catalyzes the irreversible hydrolytic deamination of adenosine and deoxyadenosine to inosine and deoxyinosine respectively, plays an important role in the degradation of adenine nucleotide and purine nucleotide salvage pathway metabolism. We investigated human epidermal adenosine deaminase activity using a radiochemical method, which enabled us to measure the adenosine deaminase activity of protein samples as small as several micrograms. We measured adenosine deaminase activity of microdissected pure epidermis of the healthy skin and the psoriatic affected and unaffected skin. It was shown that psoriatic affected epidermis had increased adenosine deaminase activity compared with the healthy epidermis (P less than 0.05) and the unaffected epidermis (P less than 0.01). There was no difference in enzyme activity between healthy and psoriatic unaffected epidermis. The increased adenosine deaminase activity in the psoriatic affected epidermis may reflect the accelerated salvage pathway of the nucleic acid metabolism probably associated with the hyperproliferative condition of the psoriatic epidermis.