SUMMARY Polyacrylamide gel electrophoresis was used to compare bacterial proteins of Brucella abortus, B. melitensis and B. suis and of the relatively new members of the genus, B. neotomae, B. canis and B. ovis. With the exception of the two repre- sentatives of a new fifth biotype of B. suis, all the protein patterns were identical. Yersinia enterocolitica IX and Y. enterocolitica NCTCI 046 I gave indistinguishable protein patterns which were distinct from those of Brucella. The results suggest that B. neotomae, B. canis and B. ovis are assigned to the correct genus but indicate that the taxonomic status of the B. suis biotype 5 representatives should be re-examined.