A simple and sensitive catalytic spectrophotometric method was developed for the determination of selenium in biological matrices. The method is based on the catalytic effect of selenium on the reaction of Methylene Blue (MB) with sodium sulfide. For a given reaction between MB and sodium sulfide, the change in the MB absorbance with time was monitored, then the time (t) required for completion of the reaction was determined, and t(-1) was calculated. A plot of t(-1) versus selenium concentration constituted the calibration graph, which was linear in the range 2.5-30 ng ml(-1) selenium. In this study, experimental parameters and the effect of interferences on determinations of selenium were examined. Tetramethylammonium hydroxide digestion was applied to blood, hair and urine samples; it was found to give the best results for urine. Then the catalytic method was applied to urine samples (84.9% recovery).