In view of the intramural infection at the hospitals and clinics and the bactrial pollution of foods and pharmaceuticals in recent years, much attetion has come to be paid to the elimination of bacteria from the air inside the rooms.<BR>For this reason, there are commercially available many types of air purifiers, but not all the purifiers which can be placed inside the room are effective in eliminating air-borne bacteria.<BR>In the present study, the authors concerned themselves with bacterial elimination by means of an A-Type air purifier.<BR>For our experimental purposes, an inlet box (B) of 1.11m³ in content and another box (C) of 0.87m³ were respectively attached to the inlet and outlet sides of this A-Type air purifier. A nebulizer made of glass, which was capable of spraying the fine 0.3 p particles was installed in front of the box (B) and, by means of this nebulizer a bacterial strain, Serratia marcescens, per 182×10⁶/ml, which was cultivated in a trypto-soy broth (in which 30g was dissolved in heated. water and sterilized to make 2, 000ml), was sprayed inside the box (B) and by activating the A-Type air purifier the bacteria were caught at two places, i. e., i) the suction inlet of the box (B) and ii) the outlet of the box (C) by use of an SY pinhole sampler.<BR>For measurement purposes, three different wind velocities of 8.7m³/min (main switch I), 9.1m³/min (main switch II) and 9.9m³/min (main switch III) were used. Moreove, the filter, special filter, activated carbon and sterilized bacterial container, which were the components of the furifier, were taken apart separately and each of them was subjected to careful examinations to measure the effect of bacterial elimination on these parts.<BR>As a result of the present study, the authors arrived at the following conclusions:<BR>1. In an experimental study with the use of Serratia marcescens, the amount of bacterial elimination by an A-Type air purifier was about 92%.<BR>2. Of various the components of the air purifier studied, the electrical dust collector had the highest effectiveness, 86.7%, followed in order by the filter with 45.7% and the sterilizing lamp with 18.7%.<BR>3. Since the effect of the sterilizing lamp was more or less similar to the 17.6% effectiveness when all the other inside components were removed, the effect of the sterilizing lamp alone was far from adequate.<BR>4. The filter installed in the present air purifier was 45% effective in eliminating bacteria.<BR>It is necessary after removing the bacteria from the air, to give due consideration to the disinfection of the bacteria still remaing inside the purifier.