Preliminary observations in a xenogeneic SCID mouse transplantation model indicated that murine epidermis overgrows human dermis from psoriatic skin but not that form normal skin. To investigate the effect of peripheral blood mononuclear cells on the differentiation of murine keratinocytes, we transplanted involved and uninvolved full-thickness skin from patients with psoriasis onto SCID mice and followed this with repeated subcutaneous injections of cells suspended in patient serum. After 6 weeks grafts were analysed morphologically and immunohistochemically. The epidermis in grafts from clinically uninvolved skin appeared normal. The persistence of a psoriasiform epidermis was noted in all grafts from affected sites despite a lack of lymphocytic infiltration. Staining for human and mouse MHC class I antigens revealed the murine origin of keratinocytes forming the psoriasiform epidermis, while the human dermis was retained. Our observations indicate that the defect underlying the pathogenesis of psoriasis is most likely located in the dermal rather than the epidermal compartment. This xenogeneic transplantation model may be useful for future studies of the pathogenesis and treatment of psoriasis.