Objectives Yunnanmycin (YNM), a novel pyrimidine nucleoside antibiotic with a sarcosylserine dipeptide side chain produced by a Streptomyces strain isolated from Guan-Ping Nature Conservatory Zone of Yunnan Province in China, was examined for its antitumor activity.Design and Methods The cytotoxicity of YNM was determined by clonogenic assay and the therapeutic effects were evaluated by transplantable tumor models in mice.Results The IC50 of YNM for KB cells was 3 μg/ml. YNM markedly suppressed the growth of sc inoculated murine solid tumors including hepatoma H22 and sarcoma 180. At tolerable doses of 120 mg/kg, po, 2, inhibition rates on H22 and S180 tumors reached respectively 83% and 81%. Compared at equitoxic dose, the inhibition rates of H22 and S180 by 5-FU, 80 mg/kg, po, 2, were 74% and 66%, respectively. Less suppression on hemopoiesis was found in YNM-treated mice than that in 5-FU-treated mice.Conclusion YNM, a novel pyrimidine nucleoside antibiotic, is highly effective against experimental solid tumors and potentially useful in cancer chemotherapy.