185 samples of amniotic fluid were obtained from 174 healthy women with uncomplicated pregnancies in the 15th-43rd week of pregnancy by abdominal amniocentesis or by puncture of the amniotic sac via an amnioscope. The concentrations of creatinine (183 cases), phospholipid phosphorus and the ratio of lecithin/sphingomyelin (L/S-ratio, 130 cases) were determined. The treatment of the amniotic fluid samples (centrifugation, filtration and lipid extraction) and the methods for the determination of L/S-ratio were examined and the accuracy and limitations of the latter method were defined and discussed. The relative amounts of lecithin and sphingomyelin as well as the L/S-ratio and creatinine concentration were closely correlated to gestational age. A L/S-ratio of ≥2.25 indicated all women to be ≥34th week of pregnancy; 64% of them had reached the 38th week. A creatinine concentration of ≥1.8 mg% corresponded to x ≥35th week of pregnancy, while 75% of the women had reached at least the 38th week. With a L/S-ratio ≥2.25 and a creatinine concentration ≥1.8 mg% of all women were ≥ the 36th week. 71% of the women ≥ the 38th week fulfilled these critiera. A combination of the two parameters seems to be of a great value in discrimination between gestational ages < the 36th week and ≥ the 38th week.