Studies of microbial secondary metabolites are being continued for the purpose of finding compounds useful in the treatment of curing cancer. Derivatives and analogs which have lower characteristic toxicities of parent antitumor antibiotics such as adriamycin, bleomycin, mitomycin, etc. are being developed. The mechanism of therapeutic action and chemical synthesis have contributed to the development of new useful bleomycins. New screening methods have always been studied and new types of interesting antitumor antibiotics such as bactobolin, oxanosine, spergualin, etc. have recently been discovered. The research area of antitumor antibiotics has been expanded to low molecular weight immunity-enhancing compounds, and bestatin, forphenicinol, arphamenine, etc. have been discovered. The effects of bestatin on human and mouse immunity systems have been studied in detail. Moreover, the study of microbial products which inhibit the generation of suppressor cells has recently been started. Oxanosine, bestatin, forphenicinol, arphamenine, etc inhibit the generation of suppressor cells. Oxanosine has strong activity. The treatment with the combination of the compounds described above will contribute to the increase of the rate of cancer cure. In this paper, the studies by H. Umezawa and his collaborators are described.