The objective of this work was to investigate the performance differences in filter media depending on whether the challenge test is conducted in the dry state or using a liquid suspension of microspheres. Comparisons of filter cut points between a pulsed airflow and a single pass liquid suspension method were used to investigate the mechanisms of filtration. Depth filtration effects were examined by comparing 2-dimensional filters (precision electroformed filters) to moderate depth filters (pillow fabric and needle felts) and a 3-dimensional depth filter (water cartridge filters). No differences were found for 2-dimensional filters but significant differences in performance were found between wet and dry analysis of 3-dimensional filters. There was little evidence of secondary filtration in the suspension method for concentrations from 0.2% down to 0.025%.