Psoriasis is a chronic inflammatory skin disease with unknown aetiology. Infiltration of inflammatory cells as the initial event in the development of new psoriatic plaques together with the defined inflamed areas of such lesions argues for an immunological disease with a local production of a causal antigen. The auto-antigen Pso p27 is a protein expressed in the skin lesions. We recently demonstrated that Pso p27 is homologous to the core amino acid sequences of squamous cell carcinoma antigens 1 and 2 (SCCA1/2) and it is apparently generated from SCCA molecules by digestion with highly specific endoproteases. In this communication we demonstrate the generation of Pso p27 from SCCA1 with extracts from psoriatic scale and even more remarkably, the generation of Pso p27 from SCCA1 in the presence of mast cell associated chymase. These findings open up for new therapeutic strategies in psoriasis and probably also in other autoimmune diseases as Pso p27 epitopes have been detected in diseased tissues from patients with various chronic inflammatory diseases.