To determine whether intestinal anaerobes are the principal agents responsible for oxidation of tritium gas (HT) in the intestine, the suppressive effects of an antibacterial drug, norfloxacin, and an antibiotic, clindamycin, on tritium uptake in rat exposed to HT were investigated. The administration of norfloxacin or clindamycin to rat was started 1, 2, 3 or 4 days prior to exposure and continued until just before exposure to HT. Following exposure to HT for 2 h, the tritium concentration of tissues and blood was determined. Tritium concentration in blood decreased in proportion to the period of drug administration. In the case of clindamycin, the tritium concentration in the body of the rat decreased to 12% that in control rat. Norfloxacin was less effective than clindamycin. When the administration of clindamycin was begun on the day of exposure and continued for 4 days, the biological half life of urinary tritium in the treated rat was shortened to 53% that in the control rat.