The stratum corneum (SC) antibodies are present in all human sera as seen by indirect immunofluorescent (IF) staining. They appear to bind in vivo to the stratum corneum of psoriatic lesions. They fix complement in vitro in a two step complement IF test system using either anti C4 or anti C3 conjugates as indicators. IF tests with proper controls showed that the SC antigen in psoriatic scales is coated not only with IgG but in a majority of the lesions also with complement. In the present studies in fully developed lesions complement was detectable in 88% of the specimens studies and in about 50% of very fresh linear lesions of unintentional Köbner type. These as well as some previously published observations afford indirect evidence for the participation of SC antibodies and the ensuing fixation of complement in the development of psoriatic lesions.