A total of 53 normal fetuses with a gestational age ranging from 15 up to 39 weeks was investigated and the radial alveolar count (RAC) was estimated as a parameter for lung maturation. Values lower than 2.0 could only be found in lungs of fetuses aged less than 18 weeks. Between 18 and 25 weeks of gestation, relatively constant levels of RAC were observed but with considerable interindividual variation. In fetuses with a gestational age of more than 25 and especially 30 weeks, a slight or rapid increase in RAC occurred respectively. Values lower than 3.0 were found up to a fetal age of less than 30 weeks and a RAC of more than 4.0 was only found in lungs of fetuses aged more than 30 weeks. Values exceeding 6.0 occurred only in fetuses at near full-term birth. Since the estimation of RAC overcomes the effects of varying degrees of alveolar collapse, such an analysis also seems to be useful for the determination for fetal age in cases of advanced putrefaction.