Abstract: Disclosed herein is an improved apparatus for conducting immunoassays. The apparatus comprises a container, and a test zone to which is bound an antibody, typically a monoclonal antibody, or which is capable of extracting cells from a fluid sample. The test zone may be a membrane, filter or a porous matrix in which microspheres to which are bound antibody are entrapped. The apparatus further comprises a liquid absorbing zone which is composed of absorbent material which acts when in contact with the test zone to induce flow through the test zone when a fluid sample is added to it. At least one port in communication with the liquid absorbing zone and the opening of the container is provided to allow gas displaced by the addition of fluid to be discharged from the container. The flow of the displaced gas being in a direction opposite to the flow of assay reagents through the liquid absorbing zone.