Analysis of 86 samples of amniotic fluid included a count of the total number of cells per cubic millimeter, the percent of fatty cells, and the lecithin/sphingomyelin ratio. In the period of gestation studied, 35 to 43 weeks, the average rise in the lecithin/sphingomyelin ratio with each week of gestation was only 0.22 (r=0.27, P<0.02), but the correlations of the lecithin/ sphingomyelin ratio with the number of fatty cells per cubic millimeter and with the nonfatty cells per cubic millimeter were highly significant (r=0.64, P<0.001; and r=0.53, P<0.001, respectively). The association of neutral fat-laden cells with the increase in lecithin could be due to a common origin from the skin of the fetus. Alternatively, exfoliation of fatty and nonfatty cells may be stimulated by the same mechanism that causes production of lung surfactant. It is suggested that surfactant lecithin becomes adsorbed onto the surface of the cells in amniotic fluid.