Infiltrate in cutaneous T-cell lymphomas (CTCLs) is composed mainly of CD4 helper cells with a phenotype very similar to that of benign cutaneous lymphoid infiltrate. MY7 (CD13) is a monoclonal antibody that is normally expressed on peripheral granulocytes and monocytes but also cross-reacts with an antigen expressed on epidermal basal cells. We studied MY7 expression on basal cells of the epidermis and CD4 cell infiltrate in 34 CTCLs, 11 pseudolymphomas, and 29 other benign cutaneous lesions. An indirect immunofluorescence technique with double labeling and an immunoperoxidase technique were used. We found that in benign inflammatory infiltrate, less than 10% of CD4 cells expressed MY7 antigen associated with normal MY7 monoclonal antibody labeling of basal cells, whereas in CTCLs more than 50% of CD4 tumoral cells in dermis expressed MY7 antigens; however, basal cells were MY7 negative. Thus, it is demonstrated that MY7 monoclonal antibody with its double modulation on epidermis (basal cells) and dermis (CD4 cells) has diagnostic value for differentiating CTCLs with CD4+ MY7+ tumor cells in dermis and MY7-negative basal cells from benign inflammatory lesions with CD4+ MY7- cells in dermis and MY7-positive basal cells. This modulation of MY7 labeling could be related to the secretion of epidermal cytokines.