A collection of Aeromonas strains of different origins were characterized for isolates expressing the O:34 somatic antigen. Of over 200 strains tested, approximately 14% belonged to serogroup O:34 with >85% of these strains identified as A. hydrophila regardless of source. A subset of 14 A. hydrophila O:34 strains were further analyzed for a number of structural and pathogenic features. Most O:34 strains expressed similar whole-cell protein profiles with regards to minor bands, but major band differences were noted in outer membrane proteins (OMPs) migrating between the 31K and 58K region, OMP profiles could be subdivided into three distinct patterns. All O:34 strains expressed a heterogeneous O polysaccharide side chain profile in their lipopolysaccharide (LPS), although some variation in the electrophoretic migration of lower and higher molecular weight LPS bands was noted, Polyclonal antisera raised against a 45-K OMP-associated protein of one O:34 strain (AH-195) reacted in immunoblot assays with a major 43 to 46-K OMP in 11 of 14 (79%) O:34 strains tested, Most O:34 strains (69%) were found to be pathogenic in mice with LD-50 values (i.p.) of <1.0 x 10(7) CFU; pathogenicity appeared to correlate best with elevated protease activity. The collective results suggest significant differences in both structural and pathogenic properties between some members of the O:34 group originating from human and nonhuman (fish, water) sources.