An improved method for the determination of selenium in biological materials has been developed. This work both extends and validates the procedure of Vézina and Bleau (J. Chromatog. 426, 385-391, 1988) which is based on high-performance liquid chromatographic determination of the fluorophore formed by reaction of Se(IV) with 2,3-diaminonaphthalene. The mass detection limit is 48 pg selenium (3 sigma) and the concentration detection limits are 48 parts per trillion in biological fluids and 120 to 480 parts per trillion in dried biological materials. The linear dynamic range of the method has been extended up to approximately 800 ng. Relative standard deviations of 9.4 to 2.7% were observed in repeated analyses of standards in the range of 0.5 to 500 ng. The proposed method was validated with respect to 23 biological reference materials spanning an 1800-fold range of selenium concentrations and was found to be free of significant constant or proportional biases despite greatly different matrix compositions. This method offers an unsurpassed combination of sensitivity, accuracy, linear dynamic range, and freedom from matrix interferences and may be considered a reference method for the reliable determination of selenium in biological materials.