Immunofluorescence (IF) studies by the direct and indirect methods demonstrate immunoglobulins and complement bound in vivo in psoriatic scales. The IF pattern is comparable to that of stratum corneum antibodies (SCAb) bound in vitro on specific substrate, as visualized by the indirect IF method. Formation of immune complexes can be responsible for the "squirting papilla" phenomenon, and conversion of the stratum corneum - which is normally an inaccessible antigen - into its reactive form seems to be brought about by proteases of polymorphonuclear leukocytes. Stimulation of protease production by polymorphonuclears appears to be an important factor in the pathogenesis of psoriasis. The stratum corneum of the epidermis is probably the target, and becomes an antigen for SCAb present in the circulation.