The O-polysaccharide of Providencia stuartii O33 was obtained by mild acid degradation of the lipopolysaccharide and the following structure of the tetrasaccharide repeating unit was established:->6)-α-d-GlcpNAc-(1->4)-α-d-GalpA-(1->3)-α-d-GlcpNAc-(1->3)-@ b-d-Quip4N(Ac-d-Asp)-(1->, where d-Qui4N(Ac-d-Asp) is 4-(N-acetyl-d-aspart-4-yl)amino-4,6-dideoxy-d-glucose. Structural studies were performed using sugar and methylation analyses and NMR spectroscopy, including conventional 2D 1 H, 1 H COSY, TOCSY, NOESY and 1 H, 1 3 C HSQC experiments as well as COSY and NOESY experiments in an H 2 O-D 2 O mixture to reveal correlations for NH protons. The O-polysaccharide of P. stuartii O33 shares an α-d-Glcp NAc-(1->3)-β-d-Quip4N(Ac-d-Asp) epitope with that of Proteus mirabilis O38, which seems to be responsible for a marked serological cross-reactivity of anti-P. stuartii O33 serum with the lipopolysaccharide of the latter bacterium. P. stuartii O33 is serologically related also to P. stuartii O4, whose O-polysaccharide contains a lateral β-d-Qui4N(Ac-l-Asp) residue.