Structures of the O‐specific polysaccharide chains of lipopolysaccharides from Proteusgroup OX strains (serogroups O1–O3) used as antigens in Weil‐Felix test for diagnosis of rickettsiosis, were established. From them, the acid‐labile polysaccharide of Proteus vulgarisOX19 (O1) is built up of the following branched pentasaccharide repeating units connected via a phosphate group: where QuiNAc stands for 2‐acetamido‐2,6‐dideoxyglucose (N‐acetylquinovosamine). The basis of serospecificity of the Proteusgroup OX antigens and their cross‐reactivity with human anti‐rickettsial antibodies is discussed.