The genus Providencia belongs to the Enterobacteriaceae family and currently consists of five species: P.alcalifaciens, P.heimbachae, P.rettgerii, P.rustigianii and P.stuartii .The serological classification scheme of P.alcalifaciens, P.rustigianii and P.stuartii includes 63 O-serogroups and 30 H-serogroups. The O-antigenic specificity is defined by the structure of the O-antigen(O-specific polysaccharide ?OPS), a part of the lipopolysaccharide (LPS,endotoxin),one of the major components of the outer membrane of Gram-negative bacteria and an important virulence factor of these bacteria. Among the bacteria of the Enterobacteriaceae family, the genus Providencia is one of the least studied in respect to its LPS structure and antigenic specificity. Studies of the chemical structures and the serological specificity of the O-antigens aim at the elucidation of the molecular basis of the serological classification of Providenciasp. LPS and alkali-treated LPS of P.alcalifaciens O23 and serologically related P.rustigianii O14, P.mirabilis O13 and P.myxofaciens as well as O-antiserum against P.alcalifaciens O23 were used. Serological characterization of P.alcalifaciens O23 O-specific polysaccharide was done by use enzyme immunosorbent assay (EIA), passive hemolysis test (PHT)as well as by inhibition and sodium deoxycholate polyacrylamide gel electrophoresis (DOC-PAGE)of LPS and Western blot. The OPS of P.alcalifaciens,O23,contains an N-(D-glucuronoyl)-N-[(R)-1-carboxyethyl ]-L- lysine residue (GlcAAlaLys).The LPS of P.alcalifaciens,O23,and other LPSs containing AlaLys from Providencia and Proteus strains were tested with rabbit anti-P.alcalifiaciens O23 serum. The serological data showed that a GlcAAlaLys-associated epitope plays a role as an antigenic determinant in the P.alcalifaciens O23 OPS and revealed the particular importance of glucuronic acid and the carboxyethyl group for the binding of O23-specific antibodies.