The antibiotic prumycin has been identified as 4-D-alanylamino-2-amino-2,4-dideoxy-L-arabinose (1). 1H and 13C N.m.r. data for prumycin and related compounds, particularly the methyl glycoside of O-acetyl-NN′-bisbenzyl-oxycarbonylprumycin (6β) and NN′-diacetylprumycin (3), were major factors in establishing the structure. The absolute configuration was derived by comparing the optical rotation of prumycin with those of other L-arabinose derivatives.