A gamma-sterilizable nutrient medium based on casein soya peptone agar for the detection of microorganisms in hydrogen peroxide-bearing air or on hydrogen peroxide-bearing surfaces, with a content of between 2 and 10% by weight of sodium thioglycolate, between 5 and 20% by weight of sodium thiosulfate and between 10 and 30% by weight of sodium disulfite in each case with respect to the agar. Preferably the agar used is microbial content test agar and the nutrient medium may contain between 0.1 and 0.25% by weight of sodium pyruvate with respect to the agar. If bromocresol purple and bromocresol violet are used as pH-indicators the nutrient medium may also contain polyvinylpyrrolidone.