An assay cartridge has a base member (26) that defines at least two wells (30, 32, 34, 36, 38), a pipette (108, 110) positionable in at least one of the wells and a cap member (86) arranged to carry the pipette. The cap member can be releasably fastened to the base member. An extension member (28) defines at least one further well (40, 42, 44) and can be fastened to the base member such that the pipette is then positionable in at least one of the wells of the base and in the further well of the extension member.