Neothramycins A and B have weak activities against some bacteria and fungi. A marked prolongation in the survival period of mice implanted with leukemia L-1210 cells was observed after i.p. treatment with neothramycin A or B. In treatment with daily i.p. doses of 25 .apprx. 100 .mu.g of neothramycin A or B per mouse for 10 days, more than 200% of prolongation in the survival period of mice inoculated with Ehrlich ascites carcinoma cells was observed. Neothramycins A and B also inhibited multiplications of Yoshida rat sarcoma cells and C3H cells transformed by SV40 in tissue cultures. Acute LD50 of neothramycin A or B in mice was 20 .apprx. 30 mg/kg by i.v. injection and 20 .apprx. 30 mg/kg by i.p. injection.