An assay apparatus comprising: i) an assay cartridge (52, 53) comprising at least one well (57-62) and a pipette (50) positionable in at least one said well; ii) a holder arranged to received said cartridge; iii) drive means operable to position said pipette in selected wells of said cartridge; iv) a gas pressure applicator couplable to said pipette whereby to cause liquid flow through said membrane; and v) a radiation detector operable to detect radiation from a well of said cartridge of said cartridge or from said pipette.