Analytical data suggest that capreomycins and viomycin are members of the same closely related family of antibiotics. We recently proposed structure (I) for the tuberculostatic antibiotic viomycin and noted similarities between the physical, chemical and pharmacological properties of viomycin and the capreomycin group of antibiotics. The capreomycins were isolated initially in 1960 from Streptomyces capreolus and were shown subsequently to be a complex of four closely related components, designated capreomycins IA, IB, IIA and IIB. We have concentrated on capreomycin IB as this is the principal component of the complex. Our analytical data and molecular weight determinations on the sulphate, free from triethylamine sulphate, are consistent with the molecular formula C25H44N14O7·2H2SO4 (molecular weight 850.79) in accordance with our proposed structure (II). Titration of capreomycin IB revealed the presence of four basic groups with pKa values of 6.2, 8.2, 10.1, and 13.3. Capreomycin IB exhibits the same characteristic ultraviolet spectrum and low field signal in the nuclear magnetic resonance spectrum as that observed with viomycin, indicating the same chromophoric unit (a ureide of dehydroserine) is present in both antibiotics. Total acid hydrolysis afforded αβ diaminopropionic acid, alanine, β-lysine and capreomycidine (ratio 2:1:1:1), as well as urea, carbon dioxide and trace amounts of glycine. The sequence of the amino-acids in capreomycin IB has been determined from the evidence of end-group analyses and the structure of the dipeptides obtained from partial acid hydrolyses. This evidence is completely consistent with structure (II) for capreomycin IB. It is now apparent that viomycin and the capreomycins are members of a closely related family of antibiotics containing a dehydroserine ureide unit and a cyclized arginine system.