Eight test microorganisms (Gram-positive and Gram-negative bacteria, yeast-like fungi, moulds) and bacterial spores were treated with vaporized H2O2 at 19-22 degrees C optionally in presence of NH3 to study the biocidal activity of the gas. The disinfection efficiency increased with the increasing H2O2 concn. in the gas phase (150-2000 ppm). The addn. of NH3 resulted in an enhacement of biocidic action of H2O2 . The fungi were more resistant to the H2O2 action than the bacteria.