In the previous papers it was reported that surfactants (surface active substances) accelerate the permeability of the cell membrane. This paper describes the results of several experiments, conducted on the assumption that, when the permeability of the cell is artificially accelerated with surfactants, some of anti-tumor agents would have more ready access into cancer cells, resulting in a greater anti-tumor effect. The results may be summarized as follows. 1. In intraperitoneal injections of saponified surfactants such as Tween 80, saponin, osvan, and OX, no marked prolongation of survival can be observed, except with OX, but the similar injections of Mitomycin C combined with Tween 80 or OX clearly show an enhanced anti-tumor effect. In other words, Mitomycin C administered in conjunction with Tween 80 or OX seems to be more readily introduced into carcinoma cells. On the other hand, there is no appreciable effect in the concurrent use of Thio-TEPA with surfactants. 2. When the peritoneal cavity of the mice is washed with the mixed solution of Mitomycin C and Tween 80 or with that of Mitomycin C and OX soon after transplantation of Ehrlich ascites carcinoma (after the lapse of 4 hours and 48 hours), likewise a marked prolongation of survival has been observed. In short, clinically at the suture following the gastric cancer resection, it is reasonable to assume that the washing of the peritoneal cavity with the mixture of Mitomycin C and surfactant solution would help to bring about a better prognosis. 3. Actually, when the mixed solution of Mitomycin C and surfactant is diffusely injected at the site of the tumor of superficial type in the patient bearing mammary recurrent cancer, a dramatic effect has been atained on the tumor without injury to normal tissues. As has been described in the foregoing, anti-tumor agents, when used along with surfactants, have been verified, experimentally and clinically, to accelerate the permeabilty of the cancer cell membrane, resulting in a more ready access of the anti-tumor agent into the cancer cells.