The controversy about the presence of the pentose arabinose in brain hyaluronate was reinvestigated using modern analytical tehcnics. The purified bovine brain hyaluronate contained the neutral sugars: arabinose, 0.18%; glucose, 0.05%; and fucose, 0.22%. The confirmation of the presence of arabinose was obtained by paper and thin layer chromatography of the neutral sugars in deionized hyaluronate hydrolysate. Gas-liquid chromatography of the aldononitrile peracetate of the pentose isolated by preparative paper chromatography gave a single distinct peak, corresponding to standard arabinose on three columns packed with three different phases. Chemical ionization data and mass spectrum of the aldononitrile peracetate derivative agreed with those of the authentic arabinonitrile tetracetate. Analysis of the isolated pentose with the help of the enzymes L-arabinose isomerase and L-ribulose kinase, which are specific for their substrates, further established its identity as L-arabinose.