The potential of specifically targeting antineoplastic drugs and toxins to tumors with the use of monoclonal antibodies (MoAbs) reactive with tumor-associated antigens is currently being examined. N-Acetyl-melphalan-MoAb (N-AcMEL-MoAb) conjugates have previously been shown to have greater antitumor activity than N-AcMEL, melphalan, or MoAb alone against both subcutaneous and ascites murine thymomas in mice (1). Although this conjugate is also a highly selective tumor inhibitor in vitro, it may not reach all the tumor cells in a high concentration, and consequently larger tumors (greater than 0.4 cm2) cannot be eradicated. This conjugate is representative of many drug-MoAb conjugates in that they are unable to gain adequate access to the tumor site to exert their cytotoxic effect. To potentiate the antitumor effect of the N-AcMEL-MoAb conjugate, studies were undertaken to analyze its action in combination with recombinant human tumor necrosis factor alpha (rTNF-alpha), a monokine, capable of causing acute necrosis of syngeneic tumor transplants in mice. Treatment of mice with murine thymomas (0.4 to 0.6 cm2 in size) demonstrated that 30% of the tumors in mice receiving conjugate and rTNF-alpha partially or completely regressed, while no regressions were observed in the tumors of mice receiving N-AcMEL-anti-Ly-2.1 conjugate or rTNF-alpha alone. This and other experiments indicated that the antitumor effect and tumor localization of N-AcMEL-MoAb conjugates can be enhanced in vivo by rTNF-alpha, thereby enabling successful eradication of larger established subcutaneous murine tumors.