Resistive-pulse counting studies of amniotic fluid lamellar bodies are presented and demonstrate a strong concordance with the predictions of accepted measures of fetal lung maturity. Uncentrifuged as well as centrifuged specimens could be evaluated, because cells and debris are rejected electronically. The technique is not affected by bilirubin or debris of lysed whole blood, and only mildly by meconium. Lamellar body number density and mean lamellar body volume were determined for 161 uncentrifuged and 241 centrifuged specimens. Number density maturity criteria (40,000/microL and 26,000/microL, respectively) were shown to be highly concordant with established measures of fetal lung maturity; mean lamellar body volume did not extend this concordance. Since electronic cell counters are generally available 24 h per day and the approach requires neither centrifugation nor subjective interpretation and is rapid and inexpensive, it is proposed that determining lamellar body number density by resistive-pulse counting may be a useful initial screen for the assessment of fetal lung maturity.