This test method covers the determination of maximum pore diameter and permeability of rigid porous filters used in the laboratory for filtration or diffusion. They are applicable to filters made of sintered glass, ceramic, metal, or plastic. This test method establishes a uniform designation for maximum pore diameter and also provides a means of detecting and measuring changes which occur through continued use. Maximum pore diameter is determined by immersing the filter in a suitable test liquid and applying air pressure until the first bubble of air passes through the filter. The maximum pore diameter is calculated from the surface tension of the test liquid and the applied pressure. Permeability is determined by measuring the flow of air through the filter when subjected to a pressure differential.