Three crystalline basic antibiotics have been isolated from fermentation broths of an actinomycete designated as Streptomyces plicatus. One of these antibiotics is amicetin, while the other two, designated as plicacetin and bamicetin, are novel. The three entities readily can be differentiated from each other by paper chromatographic and solvent extraction procedures. Comparison of physical and chemical properties indicates that the three antibiotics are closely related in structure. From acid hydrolysates of amicetin (I), cytimidine (II) and the N-dimethylaminoglycoside, amicetamine (III), have been isolated. Acid degradation of bamicetin yields cytimidine and the monomethylaminoglycoside, bamicetamine. Plicacetin has been synthesized from cytosamine (IV) and p-nitrobenzoyl chloride, followed by reduction of the nitro group. Other microbiologically active derivatives have been prepared and the structural requirement for activity deduced. The isolation and characterization of amicetin and two novel basic antibiotics, bamicetin and plicacetin, was described in a previous publication. This paper describes some studies on the structural interrelationships of the three antibiotics, the synthesis of plicacetin from the alkaline degradative fragment, cytosamine, and the preparation of some microbiologically active derivatives.