Inclusion Criteria:

1. Patients must be at least 12 years old.

2. Patients will be eligible regardless of their type of disease (malignant or
non-malignant), type of donor (HLA matched related, mismatched related or unrelated
donors), type of hematopoietic cell source (unstimulated marrow, cytokine stimulated
marrow, cytokine stimulated peripheral blood or umbilical cord blood), or GVHD
prophylaxis.

3. Patients must receive a myeloablative or moderately intensive reduced intensity (at
least 8 mg/kg oral busulfan (or the equivalent IV dose), or at least 100 mg/m2 of
Melphalan , or at least 100 mg/kg of cyclophosphamide, or at least 500 cGy of TBI)
conditioning regimen.

Exclusion Criteria:

1. Age under 12 years.

2. Known hypersensitivity to rifaximin, or other rifamycin antimicrobial agents.

3. Minimally toxic conditioning regimen (e.g. low dose TBI based). Since these regimens
induce minimal myelosuppression and gut injury, patients receiving them probably
stand little to gain from antibiotic prophylaxis.

4. Patients with documented severe active infection (viral, bacterial, fungal,
protozoal) will not be eligible.

5. Patients with treatment unresponsive hematologic malignant diseases (based on an
assessment done within two weeks of the start of conditioning therapy).