The phenotypes of mononuclear cells in 16 patients with mycosis fungoides (MF) (8 were in tumor stage and 8 in pretumor stage) were tested by ABC immunochemical technique. The results showed that the percentage of CD38 and Leu6(+) cells in tumor stage was higher than that in pretumor stage. CD38 was identified to be a marker of the immature lymphocytes. The change of CD38 might be helpful in staging MF and predicting the prognosis of the disease. In addition, Leu6(+) cells, oval or round in shape and without dendritic processes were found in tumor stage, suggesting that they might be immature lymphoid cells. This feature needs further investigation.