An experimental set-up is described permitting the evaluation and quantification under controlled conditions of retention efficiencies of surgical mask materials and different types of masks, using bacterial aerosols standardized both quantitatively and qualitatively. Screening of various materials used in one-way surgical paper masks has shown that, apart from the one-layered thin models, present paper masks with bacterial transmittance under 1% are far superior to the muslin masks of the past. For meeting the aseptical requirements in hospital operating rooms, adaptable masks (green exterior) provided with multi-layered polyester filter pads have proven most suitable by their virtual impenetrability for bacteria. After several hours use and heavy wetting of the filters layers, no increase of bacterial transmittance was observed.