In the guinea pig, the epidermal Langerhans cells studied by adenosine triphosphatase and electron microscopic techniques in dinitrochlorobenzene-induced contact dermatitis showed early cellular vacuolar and granular changes and intraepidermal contact with mononuclear cells. At later periods of up to 48 hours, the Langerhans cells migrated to the surface of a thickened epidermis and were lost in the parakeratotic horny layer that was shed. Thus, the Langerhans cell probably has a macrophage-type role in the epidermal reaction of contact dermatitis, and as the sponglosis and the inflammatory reaction develop, these cells are shed with the degenerating keratinocytes.