A newly constructed test system, utilising two optical particle counters, has been used to determine the fractional efficiency of fibrous filter media for bioaerosols (bacteria, fungal spores and pollen) and non-biological particles (PSL and limestone). The collection efficiency of coarse and fine dust filters for bioaerosols was comparable with that of non-biological particles of the same aerodynamic diameter. Particle bounce occurred for particles > 5 mu m in diameter at filtration velocities > 1 m s(-1) when fractional efficiencies for biological and limestone particles declined with increasing particle size and the efficiency for bioaerosols is higher than that for limestone particles. Experimentally determined fractional efficiencies agreed well with model calculations that take filter inhomogeneities into account while the adhesion probability of biological particles could be described by a formula initially derived for non-biological particles.