Abstract: An apparatus for measuring precise micro quantities of fluid samples is disclosed which uses a precision, disposable, self-filling capillary tube on which a holder is mounted between its ends. A plastic adaptor is mounted on one of the ends so that when a protective shield is mounted on the holder the plastic adaptor makes sealing contact with the inner surfaces of the shield. The portion of the shield extending above the adaptor forms an overflow chamber which is utilized for mixing the fluid with a diluent. A vent is provided in the closed end of the shield so that the capillary tube is open to atmospheric pressure to permit the tube to fill with the sample while the shield is mounted in place on the holder.