Amniotic fluid lecithin (L) sphingomyelin (S) ratio {L/S) is commonly determined by thin layer chromatography (TLC), acid spray. and high temperature charring, after Cluck who initially employed a quantitative method for L and S using transmission densitometry [l]. Although Cluck et al found that the ‘observed L/S of 2: 1 by reflectance de~sitometry actually represented a ‘ratio of concentration’ or actual L/S of 4: 1 [Z], control of conditions and thorough evaluation resulted in clinically meaningful data. An ‘observed’ ratio of 2 or more was found to represent maturity of the fetal surfactant system. Cluck did not comment on the prior observation that charring be carried out at lower temperatures, or the recommendation that reference standards be used whenever available f 1, op tit 3, 4] but chose to char at 280°C and ‘avoid standard curves, measurement constants, or correction factors’. Sarkozi et al [5] noted that when lipid concentrations were plotted against char density, the slopes of lecithin and sphingomyelin were different and both varied from plate to plate. Curiously, the plot of observed L/S versus the ‘ratio of concentration’ is quite linear. The observation that the observed L/S of 2: 1 reflected a ‘ratio of concentration’ of nearly 6: 1, compared to Gluck’s 4: 1, went without comment. Fainter [6] charred at less than 100°C I?]+ and obtained agreement between observed L/S and the ‘ratio of concentrations’ of standards. This study indicated an expected amniotic fluid L/S of 2 at 35 weeks gestation, and the absence of respiratory distress syndrome (RDS) if the L/S was greater than 1.5 in normal deliveries, or greater than 2 in deliveries by cesearean section. Thus, some report agreement between observed and actual L/S of standards (61, some do not [2,5], while others recommend that standards not be used for quantitation at all [g]. In addition, other phospholipids may interfere [9] because of migration with L and S. Yet, many agree that an L/S of 2 or greater indicates fetal lung maturity. Evaluation of several L/S methods in our laboratory indicated poor ~thin-run reproducibility, and charring conditions were suspected. It was desired to determine the effect of charring temperature on observed L/S.