Amniotic fluid α2-antitrypsin values were determined by radial immunodiffusion of 317 samples obtained between the 10th and 43rd weeks of gestation in normal pregnant patients and those with complications. Serial determinations of α2-antitrypsin values revealed that in normal patients values decreased as pregnancy advanced while vlaues increased as pregnancy advanced in cases in which there was severe Rh-allommunization and hydramnios. A wide range of values was observed for each week of gestation. Although there was a definite tendency for values to decrease as pregnancy advanced (60.8 mg/100 ml before 30 weeks and 17.9/100 ml after 40 weeks), the method must be regarded as unreliable for determining fetal maturity in cliincal practice. In the case of newborn infants with hyaline membrane disease, no decrease in amniotic fluid α2-antitrypsin values had been found.