The structures of seldomycin factors 1 and 2 have been determined by consideration of chemical degradation and spectral properties. Factor 1, also known as XK-88-1, is shown to be 6-0-(2-amino-2-deoxy-a-D-xylopyranosyl) paromamine (1) and factor 2, also known as XK-88-2, is shown to be 4'-deoxy-neamine (2). Mass spectral evidence has been obtained that suggests the most probable structure for seldomycin factor 3, also known as XK-88-3, is 6'-amino-6'-deoxyseldomycin factor 1 (12).The seldomycin factors are a group of aminoglycoside antibiotics isolated from the fermentation of a novel species of Streptomycetes, S. hofunensis. The mixture of antibiotics consists of four compounds previously designated XK-88-1, -2, -3, and -5 now generically named seldomycin factors 1, 2, 3 and 5. Previous papers in this series1,2) have presented the fermentation, isolation and general characterization of these compounds. This paper will outline the evidence for the structures of seldomycin factors 1 and 2 and offer a suggestion for the probable structure of factor 3. The following paper in this series3) presents the structural evidence for seldomycin factor 5, therapeutically the most significant. Structure of Seldomycin Factor 1The 100 MHz PMR spectrum of a D2O solution of seldomycin factor 1 free base (1) (Fig. 1) indicated that this material is a pseudo-trisaccharide since two anomeric resonances are visible at low field. The anomeric coupling constants are both 4.0 Hz compatible with the x-D anomeric configuration generally found in other pseudo-trisaccharide aminoglycosides. Spin decoupling experiments revealed that both anomerics are affected by irradiation at 3.23 ppm. These relatively high field chemical shifts of H-2' and H-2" require that both are shielded by amine nitrogens and indicate that both sugars are 2-deoxy-2-amino derivatives.