The structure of galtamycinone, a chromophore moiety of galtamycin, a new antitumor antibiotic was determined by spectral analysis, i.e. UV, NMR and mass spectrometry. Galtamycinone was shown to be 1,4,6-trioxy-10 [4 (e), 5 (e)-dioxy-6 (e)-methyl-tetrahydropyran-2(e)-yl]-8-methyl tetracendion-5,12. It represents a new type of anthracycline chromophores containing a C-glycoside bond in its molecule.