In 27 women with fetuses affected by beta-thalassemia major, termination of gestation between 19 and 21 weeks was induced by amniocentesis and intrauterine instillation of prostaglandin F2 alpha. Pharmacokinetics in maternal blood and amniotic fluid were studied after at least three doses of one of the following antibiotics and before prostaglandin F2 alpha infusion: (1) cefoxitin, 2 gm, intravenously, 1/2-hour infusion, three times per day; (2) moxalactam, 2 gm, intravenously, 1/2-hour infusion, three times per day; and (3) ceftazidime, 1 gm, intramuscularly, three times per day. Successful amniotic fluid levels effective against various pathogens implicated in maternal-fetal infections appeared at least 3 hours beyond administration of the drug and ranged between 2.3 and 6.7 micrograms/ml, 1.56 and 15 micrograms/ml, and 1.5 and 5 micrograms/ml for cefoxitin, moxalactam, and ceftazidime, respectively. Beyond the third-hour after infusion a percentage ratio of amniotic fluid to simultaneous maternal serum level of almost greater than or equal to 50 was constantly observed for all studied antibiotics. Cefoxitin serum levels were about the same as those in nonpregnant women, while moxalactam and ceftazidime serum levels were 50% lower than the expected level in normal individuals.