The performance of pulse-jet bag filters is characterised by two contradictory parameters: separation efficiency and pressure drop. Both parameters depend on the filtration time when the dust layer is created. The time between cleaning pulses – the filtration cycle – is also of great importance for the separation efficiency of the filter. This paper describes the results of experimental studies on the wood dust separation efficiency of a pilot scale pulse-jet filter during the initial phase of the process when the number of particles penetrating the filter visibly decreased.