Crude Natto Hay Bacillus extract of catalase without hydrogen peroxide showed a 22% decrease with hydrogen peroxide. With hydrogen peroxide plus potassium cyanide, a 48% decrease occurred with hydrogen peroxide, in regard to the ability to take up mercury from air saturated with mercury vapour. Purified extract without hydrogen peroxide showed a 63% decrease with hydrogen peroxide compared to hydrogen peroxide plus potassium cyanide which showed a 76% decrease with hydrogen peroxide alone. Hydrogen peroxide plus sodium azide showed a 97% decrease with hydrogen peroxide alone. The results were similar to human and mouse normal and acatalasemic erythrocytes or mouse lung and liver homogenates. Natto Hay Bacillus extract of catalase took up mercury from air saturated with mercury vapour. This indicates that the catalase-hydrogen peroxide system plays a role in mercury uptake presumably by oxidation.