Tuberculosis (TB), is one of the most important communicable diseases in developing countries including Iran. One of the side effects in treatment of this disease [by Isoniazid (INH), Rifampin (RMP) and Pirazinamide (PZA)] is liver toxicity and sometimes this problem may be conduce drug interruption, to save the patient from hepatotoxicity and death. On the other hand drug resistance mycobacterium is a risk which may happen with drug interruption. Prevention of liver toxicity by natural products in these patients seems to be a good strategy. We have already worked on the Salep (a plant belongs to family Orchidacea) for its protective effects on liver.1 The results of that study were hopeful. In most cases, during detoxification of drugs, microsomes in the liver causes production of toxic and active metabolites that can damage various tissues including the liver itself.2 INH, is used as a drug for treatment of active and latent tuberculosis along with other first line antituberculosis drugs. The most common side effect of this drug is elevation of liver enzymes level in blood which may be conduce to hepatitis that is a dangress complication in the patient.3 Hepatic toxicity caused by isoniazid can appear as cellular necrosis, steatosis (accumulation of fats), or both. Metabolites of this drug also have toxic effects on liver cells.4 Hydrazine is one of the most important metabolites of isoniazid. In animalmodels, a statistically significant positive correlation has been reported between plasma levels of hydrazine and severity of hepatic cell destruction caused by treatment with isoniazid.5 Moreover, other studies have proved acetyl hydrazine is another destructive agent of hepatic cells when isoniazid is used for treatment.6 Fortunately, antioxidants can prevent the destructive effects of toxins and various drugs on the liver tissue.7e9 Plants have always been considered one of the main options for treating poisoned livers because they are available as natural sources of antioxidants. Orchid or Dactylorhiza lancibracteata (C. Koch) Renz, formerly named Orchis maculate L. (or Salep in general dialect), belongs to the family Orchidaceae, has various species, and grows almost everywhere in the world. Its root nodules, can be harvested in early summer, and its medicinal properties may last for about two years.10,11 This plant contains compounds including nitrogenous materials, starch, proteins, sugars, hydroxybenzaldehydes, ferulic acid, quercetin, daucosterol, cirsilineol, steroids, and glucomannan.12,13 In traditional medicine, orchid is prescribed as ointment for chest pain, and for treating chest and intestinal disorders, tuberculosis, diarrhea, Parkinson’s disease, and cancer.14,15 Evaluation of the extracts of the roots of orchid revealed that there are numerous compounds in it; such as glucomannan and polyphenols, which possess strong antioxidant properties. In one of our previous studies we showed that this plant has protective effects against liver toxicity overall.1 Therefore, this research was conducted in rats; to study the preventive effects of aqueous extract of this plant against liver toxicity caused by isoniazid.