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Studies by Liehr et al. suggest that endotoxins are important in the pathogenesis of galactosamine hepatitis (Gal-N hepatitis) in rats. Lactulose (9.1 gm per kg per day) prevents hepatic lesions induced by Gal-N; an antiendotoxin effect of lactulose is postulated. However, commercial preparations of lactulose are contaminated with galactose, which shows a competitive action to Gal-N. To analyze the effect of galactose, male Wistar rats were pretreated with lactulose (Duphalac, 9.1 gm per kg per day) and given Gal-N (375 mg per kg i.p.). After 24 hr, serum was analyzed for glutamic pyruvate transaminase, glutamate dehydrogenase, and sorbitol dehydrogenase activities. Pretreatment with Duphalac, even 1 hr before Gal-N, abolished toxicity. Duphalac contains 10 gm galactose per 100 ml. Galactose was given in a similar concentration and similar inhibition occurred. Pretreatment with purified lactulose (9.1 gm per kg for 5 days) diminished the effects of Gal-N but did not normalize enzyme concentrations. Because small doses of galactose (80 and 300 mg per kg) showed similar inhibitory effects, we conclude that the protective effect of commercial lactulose preparations is mainly due to galactose contamination and not to an antiendotoxin effect.