Munro's microabscess of psoriasis has been generally considered to be composed of polymorphonuclear neutrophils (PMNs). However, the cell-components of the microabscess has not been fully clarified. To identify the presence of mononuclear cells (MNCs) in Munro's microabscess and the subcorneal pustule of pustular psoriasis, we observed psoriatic lesions histologically and immunohistologically. Morphologically a few MNCs were found among PMNs and they seemed to include Langerhans cells (CD1a+), helper/inducer T cells (CD4+, CD5+), and macrophages (CD14+) which expressed HLA-DR. Some of them seemed to produce gamma-interferon (IFN-gamma), interleukin 6 (IL-6) and IL-8. The keratinocytes surrounding the microabscess also had IL-8 and some epidermal cells of the proliferated epidermal papillae also expressed IL-6 and IL-8 in psoriatic lesions. The production of IL-6 and IL-8 is considered to effect both chemotaxis and the proliferation of epidermal cells. The presence of these cytokines also suggests that MNCs exist in Munro's microabscess and that they might contribute to the microabscess formation of psoriatic lesions.