The relationship between amniotic fluid optical density at 650 nm (OD650) and the lecithin: sphingomyelin (L:S) ratio was compared in 174 samples of amniotic fluid from 143 patients. An OD650greater than 0.15 accurately predicted an L:S ratio greater than 2.0 with an overall reliability of 67.9%. There were 19.5% false-negative and 12.6% falsepositive predictions. The OD650was found to have close correlation with the number of fetal squames in suspension in amniotic fluid. Pure surfactant phospholipid standards were found to exhibit negligible absorbance at 650 nm. OD650may be more closely related to fetal cutaneous maturity than to pulmonary surfactant content and bears only an indirect relationship with pulmonary preparedness for independent existence. The OD650test does not appear to be sufficiently reliable and specific to replace phospholipid determination as a data base upon which to make clinical decisions regarding fetal pulmonary surfactant maturity.