Brucella melitensis and B. ovis outer membrane blebs contained a protein displaying a temperature-dependent molecular mass upshift from 25 kDa to 30 kDa. A fraction of the protein tightly bound to LPS did not show the molecular mass upshift which was also blocked by exposure of the protein to Zwittergent 314. The B. melitensis heat-modifiable protein and Escherichia coli OmpA shared antigenic determinants. These data indicate that the Brucella group 3 outer membrane proteins belonged to the OmpA family of proteins.