 This study investigated the effectiveness of adding reduced glutathione, GSH, to a freezing extender used during cryopreservation of bull spermatozoa and found that GSH supplementation had positive effects on embryo development for three out of four bulls. However, individual bulls and GSH concentration may affect the results. This article was authored by Kazuko Ogata, Akira Aimei, Shinya Sato, and others.