A very simple, rapid and highly sensitive method is described for determining naproxen in serum and urine. This method is based on room temperature phosphorescence of naproxen in sodium dodecylsulphate micelles, with thallium(I) providing the external heavy atom and sodium sulphite acting as the oxygen scavenger. Under the optimum and experimental conditions, the range of application is 0.09-4.5 micrograms ml-1 and the limit of detection is 0.03 micrograms ml-1. The most relevant characteristic of this method is its great selectivity, e.g. naproxen can be determined in the presence of other nonsteroidal anti-inflammatory drugs (NSAIDs). The clinical applicability of this procedure has been tested, analysing naproxen in serum and urine samples. The analytical recoveries and inter- and intra assay precision data obtained demonstrate the usefulness of this procedure when used with very complex samples.