The invention relates to a method of testing for the presence of microorganisms in a gaseous environment comprising hydrogen peroxide, comprising the following steps: (i) bringing the gaseous environment comprising the hydrogen peroxide into contact with an agar growth medium, comprising a salt of pyruvic acid, (ii) placing the growth medium in an environment favoring the development of colonies of microorganisms; (iii) determining the presence of colonies of microorganisms which may have developed during step (ii). The invention also relates to a cassette containing an agar growth medium comprising a salt of pyruvic acid.