Gram-negative bacteria of genus Proteus are common human intestinal and urinary tract pathogens. In the genus Proteus there are four clinically important named species: P. mirabilis, P. vulgaris, P. penneri, and P. hauseri, and three unnamed Proteus genomospecies: 4, 5, and 6. The clinical significance of P. penneri, described in 1982 as a new species, is poorly documented. The aim of this work is serological characterization and classification of a ceftriaxone-susceptible P. penneri S29 strain isolated from a 34-year-old patient with postneurosurgical meningitis. In this characterization we will also include a ceftriaxon-resistant strain, P. penneri R15, isolated from the same patient after 12 days' treatment with ceftriaxon and other antibiotics. Rabbit polyclonal O-antisera were obtained against these two strains and purified lipopolysaccharides (LPS) were extracted from the bacterial mass of the P. penneri S29 and R15 strains. In the serological investigations the following tests were used: enzyme immunosorbent assay (EIA), passive immunohemolysis (PIH), inhibition of these tests, absorption of rabbit O-antisera with the respective LPS, and repeated PIH, SDS/PAGE, and Western blot techniques. The serological studies of the LPS extracted from both P. penneri strains showed the identity of both preparations of O-polysaccharides from LPS.. In P. penneri S29 O-antiserum, four different types of antibodies were described and characterized. Both investigated P. penneri S29 and R15 strains were classified to the Proteus O31ab serogroup.