Hydrogen peroxide 3%, (H₂O₂), catalase positive organisms (such as Staphylococcus will evolve gas bubbles in the presence of (H₂O₂).

Useful in the presumptive identification and differentiation of many bacteria. Beta-hemolytic organisms, such as Streptococcus species (catalase-negative), Staphylococcus species (catalase-positive), and Listeria species (catalase-positive) can be differentiated by their catalase reaction.