In order to see the effects cf the drugs that elevate the serum properdin level on cancerbearing animals, pantothenic acid (PaA), GABA, orotic acid (OrA), and vitamin K were given singly to mice bearing Ehrlich ascites carcinoma or these were concurrently administered with such anticancer drugs as Thio-TEPA and mitomycin C. The results of this study are given in the following. 1. The administration of PaA or GABA alone not only prevents the fall in the serum properdin level but also prolongs the survival time of cancer bearing animals as compared with the control group not given such treatment. When these drugs are used concurrently with the anticancer agents mentioned above, the anticancer effect and the preventive effect on the diminution in the properdin level are further enhanced. 2. Orotic acid and vitamin K have been found to have not much effect as to prolong the survival time nor to prevent the fall in the serum properdin level, whether given alone or concurrently with the anticancer agents. 3. In the concurrent use of anticancer agents, it has been demonstrated that a greater effect of prolongation of survival time as well as prevention of the fall in the serum properdin level is attained when the properdin level-raising agent is administered prior to the administration of anticancer drug. 4. Although properdin seems to be one of the anticancer factors, the anticancer effect of these drugs cannot be directly connected to the action to elevate properdin level.