The mycinamicins are 16-membered macrolide antibiotics produced by Micromonospora griseorubida, which have strong antibacterial activity against Gram-positive bacteria. In the course of searching for new antibiotics from the culture filtrates of mycinamicin-producing strains, we discovered four new minor components designated mycinamicins X (1), X' (2), XI (3), and XI' (4). These compounds were isolated by adsorption on Diaion HP-20 resin, followed by washing with water and elution with methanol, and obtained as white amorphous powders. From physico-chemical properties (molecular formula, melting point, specific rotation, UV, IR), spectroscopic data (13C NMR, FAB-MS, 1H NMR), and chemical degradation studies (alkaline treatment to yield mycinamicins I (5) or II (6), hydrolysis to produce L-cysteine), the compounds were shown to be analogs of mycinamicins I and II with an N-acetyl-L-cysteine moiety linked to C-11. The pairs mycinamicins X/X' (1/2) and XI/XI' (3/4) were confirmed as diastereoisomers through synthesis by reacting mycinamicins I or II with N-acetyl-L-cysteine and NaHCO3 in 80% aqueous acetone.