The Sarcophaga lectin gene, an acute-phase-responsive protein of Sarcophaga, was found to be activated when the fat body was incubated in phosphate-buffered insect saline (10 mM NaH2PO4/Na2HPO4, pH 6.2, 2 mM NaHCO3, 1 mM MgCl2, 5 mM KCl, 1 mM CaCl2 and 120 mM NaCl). This activation was selectively suppressed by addition of 2-mercaptoethanol. 2-Mercaptoethanol did not affect the transcription directly, suggesting that the oxidation of the sulfhydryl group of a fat-body protein is required for activation of the Sarcophaga lectin gene. This oxidation reaction seemed to occur immediately when the fat body was incubated in vitro, and once this reaction was initiated, 2-mercaptoethanol no longer inhibited transcription of the Sarcophaga lectin gene. This gene is known to be activated when the larval-body wall of Sarcophaga is injured. It is suggested that the Sarcophaga lectin gene is activated via this novel mechanism on its acute-phase expression in vivo.