For the purpose to elucidate action mechanism of pantothenic acid (PaA) and γ-aminobutyric acid (GABA) that possess the ability to raise the serum properdin level and show anticancer effect when administered to cancer-bearing animals, a series of in vivo and in vitro experiments was carried out and the following results were obtained. 1. PaA, when give to normal mice, accelerates the liver catalase activity, and when it is administered alone or concurrently with anticancer agent to cancer-bearing mice, the catalase activity is enhanced to a still greater extent. 2. PaA, GABA, Orotic acid, Thio-TEPA, and mitomycin C have been found to accelerate the anaerobic glycolysis of Ehrlich carcinoma cells but carzinophilin rather suppresses it. 3. The effect of PaA and GABA on the serum properdin level resembles closely to that of ACTH, but differs from the effect of cortisone in that the high level of the serum properdin is sustained even after the completion of drug administration. 4. GABA, ACTH, and PaA have the ability to prolong the survival time of cancer-bearing mice, but cortisone rather aggravates cancer. 5. From these findings it is concluded that PaA and GABA do not act directly on tumor cells but act on the pituitary body by way of the higher nervous center while on the other hand, these drugs act on the cells of host, especially those of the reticuloendothelial system and liver, thus potentiating anticancer factors including the serum properdin level.