It has been postulated that epidermal Langerhans cells (LC) are essential for induction and elicitation of contact sensitivity (CS) to simple chemical allergens. Although the precise role of LC in CS has not been determined, recent reports have focused on a function for LC in the presentation of antigen to T lymphocytes. To investigate this possibility, the distribution of allergen on the epidermal LC of inbred strain 13 guinea pigs was observed by the immunofluorescent method using antibodies against la antigen and hapten. 2,4-Dinitrophenyl groups were found to be localized on/in Ia-positive epidermal LC of the animals following skin painting with 2,4-dinitrochlorobenzene. This finding is discussed in relation to the role of LC as antigen presenting cells in CS.