In a Weil‐Felix test, sera from patients infected with Rickettsiasp. agglutinate ProteusOX types of bacteria and Proteuslipopolysaccharide (LPS) are responsible for the cross‐reaction. Data on the character of LPS of one of the OX group strains, Proteus vulgarisOX19, are contradictory, and it remained unclear whether it has an O‐polysaccharide (OPS) and is thus LPS of the smooth type (S) or not (rough‐type LPS). Our studies showed that P. vulgarisOX19 (strain PZH‐24) produces a smooth‐type LPS that contains a long‐chain OPS, but it undergoes depolymerization during mild acid hydrolysis conventionally used for LPS delipidation and loses the serological activity. An elucidation of the complete structure of OPS demonstrated the presence of a glycosyl phosphate linkage responsible for the acid‐lability of the polysaccharide chain. In ELISA, both IgM type antibodies in a Weil‐Felix test with human anti‐Rickettsia typhisera and rabbit anti‐P. vulgarisOX19 antibodies reacted with OPS. Rabbit antibodies did not inhibit the cross‐reaction with human antibodies and thus bind to different epitopes.