Paramecium cells are usually cultured in a wheat grass powder infusion inoculated with Klebsiella pneumoniae. However, non-bacterized wheat grass powder infusion is toxic to paramecia, and bacteria-derived substance detoxifies the toxic substance. Here, the detoxifying substance from K. pneumoniae, which was found to be proteinaceous, was purified to homogeneity. The protein had an apparent molecular mass of about 200 kDa by gel filtration and 92 kDa by SDS-polyacrylamide gel electrophoresis. Although the amino acid sequence of the amino terminal region did not show a high sequence homology with any reported proteins, amino acid sequences of internal regions of the protein were nearly identical to catalase HPII from Escherichia coli. When the wheat grass powder infusion was treated at 25 °C for 1 h with commercially available catalase from bovine liver, the toxicity of the infusion against paramecia was completely abolished. The initial concentration of hydrogen peroxide in the wheat grass powder infusion was about 30 μM and was completely decomposed by the catalase treatment. Therefore, the toxic substance in the wheat grass powder infusion and the detoxifying substance from K. pneumoniae are considered as hydrogen peroxide and catalase, respectively.