The structures of new antitumor antibiotics, glidobactins A (Ia), B (Ib) and C (Ic) were elucidated by a combination of chemical and enzymatic degradations and spectral analyses. They have in common a cyclized tripeptide nucleus composed of L-threonine, 4(S)-amino-2(E)-pentenoic acid and erythro-4-hydroxy-L-lysine, and differ from each other in the unsaturated fatty acid moiety attached to the peptide.