A novel nucleoside antibiotic, Sch 36605, has been isolated from the culture filtrate of a Streptomycete sp. All physico-chemical and spectroscopic data presented indicate this compound is a new blasticidin compound. A second closely related component, Sch 36606, was isolated together with Sch 36605 and identified as leucylblasticidin S. Sch 36605 (1) is a water-soluble basic compound with MP >230°C (dec), [α]²⁶ᴰ +58.2° (c 0.1, H₂O), IR (KBr) cm⁻¹ 3300 (br, NH and OH), 1650 (CONH), 1000-1150, UV λmax (H₂O, pH 7) 272 nm (ε 8350), and λmax (0.01 N HCl) 279 nm (ε 11240). Its molecular weight (565) and formula C₂₄H₃₉N₉O₇ were established by FAB-MS (M+H⁺ m/z 566). Acidic hydrolysis gave 5-hydroxymethylcytosine and leucine, confirming leucine linkage via the C-carboxy terminus. ¹H and ¹³C NMR (including INEPT) data supported the proposed structure. Specific rotations of blastidic acid from Sch 36605, Sch 36606, and blasticidin S indicated L-configuration for blastidic acid; leucine's L-configuration was confirmed by HPLC with GITC derivative. CD spectra (negative Cotton effect ~270 nm, consistent with blasticidin S) established the pyran-3-ene ring's β-D configuration. Thus, Sch 36605 was identified as 4-[3-(2-amino-4-methylvaleramido)-5-(1-methylguanidino)valeramido]-1-(5-hydroxymethyl-4-amino-2-oxo-1-(2H)-pyrimidinyl)-1,2,3,4-tetradeoxy-D-erythrohex-2-enopyranuronic acid.