The mycinamicins are 16-membered macrolide antibiotics produced by Micromonospora griseorubida, which have strong antibacterial activity against Gram-positive bacteria. In the course of search for new antibiotics from the culture filtrates of mycinamicin-producing strains, we have discovered four new minor components designated mycinamicins X (1), X' (2), XI (3) and XI' (4). From physico-chemical properties, spectroscopic data and chemical degradation studies, these compounds were shown to be analogs of mycinamicins I (5) and II (6), whose structures are shown in Fig. 1.