OBJECTIVE To investigate the relationship between TNFalpha and tumor rejection induced by a single dose of melphalan in C57BL/6 mice. METHODS Different gene type mice (TNFR1(+/+), TNFR1(+/-) and TNFR1(-/-)) with the same genetic background of C57BL/6 were used in this experiment. Murine lymphoma EL4 cells were inoculated subcutaneously into the different gene type mice simultaneously. Twelve days later, 7.5 mg/kg melphalan was used intraperitoneally to treat the tumor-bearing mice with TNFR1(+/+), TNFR1(+/-) and TNFR1(-/-). The tumors in the different gene type mice were observed and recorded every one to three day. RESULTS After the treatment of 7.5 mg/kg melphalan during the first week, the tumors in the different gene type mice shrank at a similar rate. In the following 2 months, the tumors in the TNFR1(+/+) and TNFR1(+/-) C57BL/6 mice gradually shrank and were cured but most tumors in the TNFR1(-/-) C57BL/6 mice relapsed after melphalan treatment. CONCLUSIONS TNFalpha plays an important role in melphalan-induced tumor rejection. The anti-tumor effect of melphalan has no relationship with the expression of tumor necrosis factor 1 in tumor-bearing mice. TNFR1 is required to prevent or avoid the relapse of tumors in mice instead of tumor cells.