Low-dose cyclophosphamide (CY) is an immunomodulating agent that down-regulates T suppressor cell function. This study investigates postoperative immunotherapy with CY as an alternate treatment for advanced immunogenic tumors such as neuroblastoma that typically respond poorly to traditional high-dose chemotherapy. A/J mice with 1.5-cm subcutaneous C1300-neuroblastoma (C1300-NB) tumors were divided into the following treatment groups: I, untreated (n = 14); II, 85% tumor resection (n = 18); III, sham-operated (n = 18); IV, multiple-dose CY (n = 6); V, 85% resection and single-dose CY (n = 14); VI, 85% resection and multiple-dose CY (n = 14). CY (100 mg/kg, intraperitoneally) was given initially 24 hours post-operatively to groups IV, V, and VI. Groups IV and VI also received weekly maintenance doses of 25 mg/kg CY. Results showed significantly increased survival (log-rank test) in CY-treated groups (IV, V, VI) compared with control groups (I,II,III). Cures were observed only in groups receiving partial resection plus CY (V, 7%; VI, 29%). Although surgical debulking of tumor alone (II) did not enhance survival, the procedure normalized depressed total lymphocyte counts and the subpopulation of Lyt 2,3+ (T suppressor/cytolytic cells) in the immediate postoperative period during which immunotherapy with CY was instigated. This may have contributed to the success of CY immunotherapy. To characterize the tumor-host immune interaction, additional studies were performed. Results showed the following. (1) Mice cured by debulking plus CY (from groups V and VI) could not be successfully reimplanted with C1300-NB, demonstrating immunologic mediation by CY.(ABSTRACT TRUNCATED AT 250 WORDS)