The skin is a primary site injured in lupus erythematosus (LE), but it is still controversial whether the injury is due to cells of the mononuclear infiltrate and which immunocompetent cells play the major role in the development of cutaneous LE. To better characterize the role of immunocompetent cells, we performed an immunohistochemical examination of these cells in LE-like skin lesions in MRL/Mp-1pr/1pr (MRL/1pr) mice. Skin lesions in 60 female MRL/1pr mice were monitored from onset to frill development. Skin specimens from each stage were stained for epidermal Ia⁺ Langer- hans cells (Ia⁺-LC), for Thy-1⁺ dendritic epidermal cells (Thy-1⁺DEC), and for the phenotype of the mononuclear cell infiltrates. The numbers of Ia⁺-LC and Thy-1⁺DEC were decreased markedly in the skin lesions at the later stage. However, the numbers of Ia⁺-LC were increased significantly in the central portion of lesions at an early stage and in the peripheral portion of lesions later. L3T4⁺ cells were predominant, and the L3T4/Lyt-2 ratio was high in dermal infiltrates at an early stage. With advancing stage, the L3T4/ Lyt-2 ratio gradually decreased in dermal infiltrates, whereas the Thy-1.2/Lyt-2 ratio in lymph nodes was reversed. L3T4⁺ cells were especially predominant in dermal infiltrates under the epidermis with increased numbers of Ia⁺-LC. This immunohistochemical analysis of a mouse model of cutaneous LE revealed changes in immunocompetent cell populations with the evolution of skin lesions, and we conclude that Ia⁺-LC and Thy-1⁺DEC, as well as L3T4⁺ and Lyt-2⁺ cells, may play pathogenic roles in the development of skin lesions.