The placental transfer of thiamphenicol was studied in 29 subjects at term after intravenous bolus injection of 500 mg of the drug. Detectable plasma levels were present in umbilical blood and amniotic fluid as early as 5 and 18 minutes, respectively, after maternal administration. With the dosage applied, minimum concentrations required for the inhibition of many organisms causing intrauterine infections were reached in maternal and fetal plasma and in amniotic fluid. The value of thiamphenicol for the treatment of intrauterine infections is discussed.