Glucose-regulated protein 58 (GRP58)-like immunoreactivity in rat liver obtained in the evening or after fasting underwent an electrophoretic band-shift, which disappeared after phosphatase-treatment. Since mass spectrometric analysis raised a possibility that Ser150 of GRP58 is phosphorylated, an antibody against the phosphoserine150 GRP58 was generated. Immunoreactivity to this antibody was increased in the evening and after fasting. Since GRP58 was shown to interact with signal transducer and activator of transduction 3 (STAT3), a leptin-related protein, the effect of leptin was examined. Immunoreactivity to the anti-phosphoGRP58 antibody was markedly elevated after the leptin injection, indicating that Ser150 of GRP58 is phosphorylated after fasting and leptin-treatment. (c) 2005 Federation of European Biochemical Societies. Published by Elsevier B.V. All rights reserved.