A method and apparatus for filtering microorganisms from a sample and culturing the microorganisms employs a membrane filter (3) in a filter holder (10) in which holder the membrane filter is supported on an absorbent support (26) which is maintained in an expansible compressed state against the membrane filter so that any expansion of the membrane filter when wetted by a sample does not cause bubbling of the membrane filter away from its support entrapping air bubbles and so preventing supply of nutrient from culture medium supplied to the support from reaching the microorganisms on the membrane filter.