The elucidation of the structures and absolute stereochemistry of megalomicins A, B, C, and C, a group of macrolide antibiotics elaborated by Micromonospora megdomicea sp. n., is described. Megatomicin A has been shown to be (2R,3S,4S,5R,6R,8R.l OR 1 1 R.12S.13R) -3-(2,6-dideoxy-3-C-methyl-a-~-~~~o-hexopyranosyioxy)-6,l2-dihydroxy-4.6,8,10,12- hexamethyl-9-0~0- 1 1 - (2.3.6-trideoxy- 3-dimethylalmino- p-D -lyxo- hexopyranosyloxy) - 5- ( 3,4,6 - t r i d e o x y - 3 - d i met h y la m i no - p - D -x ylo - h exo p y r a n o s y I ox y ) p e n t a d e ca n - 1 3 - o I id e . M e g a I o m i c i n s B , CI, a n d C, have been shown to be derivatives of megalomicin A acylated in the 3-glycosyl system.