The phenotypes of lymphoid cells from twelve patients with mycosis fungoides (including 5 patients in the tumor stage and 7 in the pretumor stage) were determined immunohistochemically using anti-thymocyte monoclonal antibodies (HIT1, HIT2, and Leu 6). The results showed significant differences between the numbers of HIT1 and HIT2 positive cells in tumor and pretumor stage lesions, indicating that these two monoclonal antibodies may be of help in the staging of mycosis fungoides and in making a prognosis. Also, we found that some Leu 6 (+) cells in tumor stage lesions were round or oval in shape without dendritic processes, suggesting that they may be immature lymphocytes.