The mycinamicins are 16-membered macrolide antibiotics produced by Micromonospora griseorubida sp. nov., which have strong antibacterial activity against Gram-positive bacteria. The complex consists of seven components; mycinamicins I (1), II (2), III (3), IV (4), V (5), VI (6) and VII (7). During the search for new antibiotics from the fermentation broth of mycinamicin-producing strains, we have discovered a new minor component designated mycinamicin VIII (8). Recently, the same product has been obtained in much higher yield from the fermentation of a mutant strain C-7-1, isolated during ongoing mutagenic studies of M. griseorubida sp. nov. In addition, this mutant was found to accumulate protomycinolide IV (9). These compounds (8 and 9) are considered to be key intermediates in the biosynthesis of the mycinamicins. In this communication we describe the isolation and identification of the new metabolite, compound 8, which provided definitive information about the biosynthetic pathway.