When purified from rat muscle, selenoprotein W is fractionated into four forms distinguished by slightly different chromatographic behavior. Precise masses of the four forms were determined by matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry. The mass distribution of the forms (9549, 9592, 9853, and 9898 d) suggests that they occur through derivatization of the lowest mass form with two moieties of approximate masses 44 and 305 d. The apparent 305 d moiety was demonstrated to be glutathione (307 d) by reductive release from the 9853 d protein form with 1000-fold excess of dithiothreitol at 50 degrees C. Milder conditions failed to remove the glutathione. The reduction produced nearly stoichiometric amounts of free glutathione as determined by HPLC of a fluorometric derivative. HPLC retention of the protein changed to match that of the 9549 d form, and a change of mass to 9550 d was observed by MALDI mass spectrometry. The identity of the 44 d moiety is unknown. The presence of glutathione in isolated selenoprotein W may suggest its involvement in the metabolism of this tripeptide.