Lamellar bodies, produced by secretory cells in the alveolar epithelium, are the major source of surfactant phospholipid. As the fetal lung matures, the membranous content of the lamellar bodies is secreted into the alveolar spaces and passes into the amniotic fluid, from which it can be isolated in a morphologically recognisable form. A method is described for the rapid isolation of a lamellar body fraction from amniotic fluid using a small air-driven clinical ultracentrifuge. The lamellar body phospholipid content of amniotic fluid increases towards the end of gestation, but the time of onset and the rate of this increase show wide individual variation. Preliminary results suggest that the lamellar body phospholipid content of amniotic fluid may be a useful index of fetal lung maturity.