Polysialic acid capsules are the major virulence factors in Escherichia coli K1, K92, and groups B and C meningococci. The sialic acid monomers (2-keto-3-deoxy-5-acetamido-7,8,9-d -glycero -d -galacto -nonulosonic acids) comprising these homopolymeric polysaccharide chains can be selectively modified with 1,2-diamino-4,5-methylenedioxy-benzene to produce highly fluorescent quinoxalinone derivatives distinguished by their elution times during reverse phase chromatography. Here, we describe methods to release the constituent capsular polysialic acid monomers, and to detect and quantify them by sensitive fluorometry. There are relatively few 2-keto acids in bacteria, making it possible to rapidly analyze samples even without prior purification of capsular polysaccharides.