Abstract: A tissue typing tray is provided with an improved well cover. The tissue typing tray is of conventional box-like construction, with conjoining side walls and a bottom and a plurality of wells disposed on the bottom. The well cover comprises a flat member, with a downwardly depending, continuous ridge member inwardly spaced from the perimeter of the flat member and terminating in a flat surface provided with a pressure sensitive adhesive thereon. The ridge member is of the same height as the wells and encompasses the wells. Upon engagement of the well cover with the tissue typing tray, the adhesive-coated ridge member contacts the bottom of the tray, while the flat member of the well cover contacts the tops of the wells. The sealing configuration of the well cover prevents its lateral or vertical movement, thereby preventing possible loss of testing reagent from the wells or contamination of the contents of one well by the contents of another.