An apparatus for determining the filterability of fluids, in particular of transmission oils, has a test filter device (21) through which a sample amount of the fluid can flow, and a line system which forms a circulation circuit (1) for the fluid. The line system includes a foam tester (3) receiving the sample amount of the fluid. A circulating pump (23) removes fluid from the sample amount situated in the foam tester (3) and conveys it through the filter device (21) and back to the foam tester (3).