A peroxidase-antiperoxidase study using monoclonal antibodies directed against T and B lymphocytes and Langerhans cells/indeterminate cells (LC/IC) was undertaken in order to understand more clearly the changes observed in erythema multiforme. At the various stages of development, from normal skin to target lesions, the quantity of inflammatory cells differed, but in each case the number of T8+ (cytotoxic/suppressor) cells was greater than the number of T4+ (helper/inducer) cells in the epidermis, whereas the latter exceeded the former in the dermis. Concomitant with the initial epidermis changes, there was an increase in the number of T6+ (LC/IC) cells in the upper and lower epidermis. With slight to moderate basal unit destruction, the number of LC/IC in the upper epidermis exceeded those in the lower epidermis. With severe basal unit destruction, there was a loss of LC/IC in the lower epidermis as detected by T6 reactivity. In fully formed blisters, the LC/IC in the upper half of the epidermis were decreased in parallel with the degree of epidermal necrosis. The character of the lymphocytic inflammatory infiltrate and redistribution in LC/IC are similar to those findings described in allergic contact dermatitis. The clinical, histologic, and immunopathologic changes in erythema multiforme appear to be due in part to cellular immune mechanisms with the lymphocyte as the predominant effector cell, and our data suggest a possible role for LC/IC in this disorder.