The expression of mdr1 gene was measured to determine whether it plays a role in clinical resistance to chemotherapy of human malignant lymphomas. mdr1 expression was found in 4 of 9 cases resistant to chemotherapy. Expression of mdr1 was not detectable in any of 7 chemotherapy-sensitive tumors. The 2 cases of reactive lymphadenitis and the 3 samples of normal mononuclear cells did not show any expression of mdr1 gene, either. These results indicate that expression of the mdr1 gene is not always detectable in cases of malignant lymphoma resistant to chemotherapy, but the detectable expression of mdr1 gene may predict clinical resistance to chemotherapy.