The complete core structure present in the lipopolysaccharide of the R mutant R110/1959 from Proteus mirabilis (Proteus type II core) was investigated using methylation analysis and a number of degradation methods such as Smith degradation and beta-elimination. These studies combined with earlier work on a Rc-type mutant of P. mirabilis O28 (R4/O28) which shares the same inner core region, allowed formulation of the complete core structure of the Proteus type II core as shown in Scheme 1. (formula; see text) A characteristic feature of the Proteus core of type II is the presence of two units of D-galacturonic acid (DGalA); one in terminal, the other one in a chain-linked position. In addition, the presence of the two isomers of glycero-D-manno-heptose (LDHep and DDHep) and the lack of galactose are conspicuous. DDHep occupies a terminal position in the external core region, whereas the three units of LDHep in addition to dOclA form, as in other enterobacterial core types, the internal core region. The taxonomic significance of the presence of DGalA in the Proteus type II core, but also in all R cores of other Proteeae investigated so far, will be discussed.