Filtron-X is a ready-to-use liquid scintillation solution which completely dissolves Millipore-type filters (cellulose acetate, cellulose nitrate and mixed ester), solubilizing a wide variety of samples contained on the filter. Filtron-X contains all the necessary phosphors and solvents to yield counting efficiencies unattainable by any other means. The use of Filtron-X assures full 4π geometry counting of samples. Improvement in counting efficiency is most notable when tritium is used as the label since absorption of the emitted beta ray by the filter is completely avoided. Filtron-X will solubilize the sample assuring homogeneous counting. For a completely dry filter it may be necessary to moisten the filter with one or two drops of distilled water before placing in Filtron-X. Simply add filter to 10ml of Filtron-X, allow to stand at room temperature for 15 minutes. Shake well to assure complete dissolution of filter and sample and then count.