Saframycin S, structurally the decyano-saframycin A and biosynthetically the precursor of saframycin A, is a new antibiotic and is assumed to be the active principle for the formation of the antibiotic-DNA complex to manifest its activity. In the present study on the biological activity of saframycin S, the antibiotic showed a marked activity against Ehrlich ascites tumor with the doses of 0.5 to 0.75 mg/kg/day for 10 days, and the rate of 40-day survivors was 80 to 90%. However, this antibiotic was less effective against P388 leukemia than saframycin A at the doses tested. Saframycin S also showed the highest antimicrobial activity, in particular against gram-positive bacteria, among the saframycin group antibiotics identified to date. the LD50 of saframycin S was 3.2 mg/kg (i.p.) for ddY mice. The biological activity and toxicity of saframycin S in relation to its structure were described.