The sequence of events leading to the formation of a psoriatic plaque induced by tape-stripping was studied using light and electron microscopy. Among the earliest changes noted were increased mobility of the epidermal Langerhans cells across the basement membrane, evidence of Langerhans cell-lymphocyte interaction, and increased Langerhans cells 'activity' or 'cytotoxicity'. These changes were seen as early as 2 min after stripping and remained until the development of clinical psoriasis. Collections of epidermal lymphocytes showing the features of blastoid transformation while in contact with processes from activated Langerhans cells, suggest the involvement of Ia antigens in this process. We postulate that these findings are a manifestation of an increased immune responsiveness to trauma, controlled by genes located at the HLA-D locus of the major histocompatibility complex, and mediated by enhanced cellular interactions. The appearance of basal keratinocyte herniations (BKH) at 1-3 weeks after stripping, coincided with the development of clinical psoriasis. Neutrophils made their appearance at the same time as, or slightly before, the appearance of BKH, and are suspected to play a role in the development of these structures. We believe that BKH maintain epidermal proliferation through the persistence of the epidermal-stromal interaction.