Occurrence of monocytoi B lymphocytes (MBL) in the lymph nodes of patients receiving preoperative chemotherapy for cancer was examined and compared to lymph nodes in controls who had not received chemotherapy. Number of patients receiving and not receiving preoperative chemotherapy were 3 and 10 cases in ovarian cancer, 7 and 11 in testicular cancer, and 22 and 8 in lung cancer, respectively. Chemotherapeutic agents for ovarian, testicular, and lung cancer consisted of cisplatin, Adriamycin, and cyclophosphamide; cisplatin, vinblastine, and bleomycin; and cisplatin, vindesin, and mitomycin, respectively. MBL were defined morphologically as having abundant pale cytoplasm with distinct cell borders and small nucleus. Immunohistochemistry revealed a B-cell nature of these cells, i.e., CD20-and/or MB-1 + together with negative reactivity for antibodies for T lymphocytes (CD43, CD45RO,OPD4) and macrophages (KP-1, PGM-I). Monocytoid cells in two cases showed a positive reactivity for CD43 together with CD20. The occurrence rate of MBL in patients with ovarian, lung, and testicular cancer receiving and not receiving chemotherapy was 67% (2/3) and 10% (l/lO), 59% (13/22) and 75% (6/8), and 43% (317) and 9% ( I / ] ] ) , respectively. The occurrence rate in the total patients receiving chemotherapy (56%) was significantly higher than for those not receiving chemotherapy (28%) ( P < 0.05). These findings suggest that chemotherapy-induced depressed immune function is causative for the occurrence of MBL in the lymph nodes. MBL might be found more frequently in nodes from patients who have received chemotherapy in certain settings. CJ 19% WiIey-Liss, Inc.