The structures of novel antimicrobial antibiotics, amythiamicins A, B and C, were elucidated by chemical degradations and NMR spectral analyses. The main frame from C-1 to C-41 of these antibiotics was the same as that of amythiamicin D. Ammo acid autoanalyses of amythiamicins A, B and C showed that these have another one mole of serine and proline in comparison with amythiamicin D. Stereochemistries of both amino acids were determined to be L by chiral HPLC. These seryl-prolyl residues in amythiamicins A, B and C are attached at C-41 through an oxazoline ring, amide and ester bond, respectively.