Abstract: A fluid level sensor and a washer unit are provided for an automated immonoassay processing system. A washer head (13) is vertically movable by a drive (2) above a reaction tray (8) having fluid (15) in separate cells. The level sensor comprises two electrodes arranged along a fluid path within aspirator tips (6). The electrodes are held at different electrical potentials and are effectively insulated from each other. If fluid is aspirated along the fluid path then the electrodes are bridged. Control means (1) is arranged to detect any change in conductance between the electrodes thereby indicating the presence of fluid (15). The vertical height of the washer head is determined relative to the sample thereby enabling the device to determine the level of the fluid (15).