Out of 134 Pasteurella and Actinobacillus strains studied, 132 are found to be susceptible to the O/129 vibriostatic agent with growth inhibition diameter ranging from 10 to 42 mm (86% in the range 25 to 32 mm); 46 reference strains of Enterobacteriaceae including Yersinia are resistant to the O/129 vibriostatic agent. This test can quickly differentiate between typical or atypical Enterobacteriaceae and the genus Pasteurella or Actinobacillus.