Particle electrophoresis has been used to investigate particles obtained from amniotic fluid. These particles are defined mainly as unilamellar and multilamellar liposomes by electron microscopy. A total of 172 samples of amniotic fluids from 132 pregnancies were included in this study. The electrophoretic results were compared to the total phospholipids and to neonatal outcomes in 42 cases. At least two kinds of particles from amniotic fluids with different electrophoretic mobilities (EPM) are evident. With a ratio of F greater than 1.0, the presence of electrophoretically faster particles was found to be associated with apparent fetal lung maturity. On the other hand when the ratio was F less than 1.0, this seems to indicate an immature fetal lung. Seventy-seven per cent of F values correspond to the total phospholipid content P regarding the prediction of fetal pulmonary maturity resp. immaturity. The correct prediction of fetal lung maturity was demonstrated in 42 patients delivered within 48 hours of amniotic fluid sampling. In 28 cases with a mature F value (greater than 1.0) the correct prediction of respiratory distress syndrome (RDS) was 100 per cent. Eight out of 14 patients with immature F values (less than 1.0) developed RDS. The F values were found to increase as pregnancy progressed. The electrophoretically differentiated amniotic fluid particles behave differently at low pH. Selection and electrophoretic measurement of particles were made in a subjective manner. This preliminary investigation suggests that the results correspond to those obtained by objective cell electrophoretic measurements.