S u m m a r y . This paper reports experiments on the purification of exhaust air containing organic pollutants by a new biological process using a trickle-bed reactor. Pollutant-specific microorganisms in high concentration were fixed to a suitable bed. The absorption and conversion of propionaldehyde as a model pollutant was measured by systematic variation of the gas and liquid flow rates in the reaction system. At a space velocity of 1000 h -1, it was possible to achieve conversion rates of between 68 and 96%, depending on the trickling density. The degradation capacity of the biological trickle bed is over 500 g propionaldehyde/m 3 of reactor per hour. By using a tube bundle (honeycomb tube), it was possible to ensure continuous operation of the reactor with reduced conversion and pressure loss.