Abstract: A method for washing an assay tray includes the following steps. A solid pad of absorbent material, such as a layer of cotton sandwiched between a cardboard and a nonwoven fabric, is firstly placed over the wells of an assay tray. The assay tray is then tilted or inverted together with the pad to allow the serum to drain off and be absorbed in the pad. The assay tray is then inserted in a washing device and submerged in the stationary washing liquid. Subsequently, the washing device is moved relative to the washing liquid to let the washing liquid flow freely in and out of the wells of the assay tray to thoroughly wash the beads and the wells, while the beads will be blocked from falling out of the wells by means of the special design of the washing device. The washing device has a broad opened wall which confines a network of ribs formed by intersections of transverse and longitudinal walls. The intersecting portions substantially superimpose the center of the wells respectively.