A new method is presented for differentiating amniotic fluid from cervical mucus, based on differences in the respective carbohydrate concentrations present in these fluids. Glucose and fructose are present in high concentrations in cervical mucus during pregnancy, average values being 240 and 30.4 mg/100 ml, respectively. Their concentrations in amniotic fluid, however, are low, averaging 39 and 3.3 mg/ml, respectively. Examination of 215 women between 36 and 42 weeks' gestation, using either glucose or fructose values, revealed a high degree of discrimination between amniotic fluid and cervical mucus. The best results, however, were obtained by employing the calculated linear sum 3.32 log glucose plus log fructose for each sample. Inaccuracy rates were low in comparison to those of other diagnostic methods, such as pH values, ferning, and Nile blue sulfate staining. A further advantage of the method is that it remains fairly accurate even when the time interval between rupture of the membranes and the investigation exceeds 24 hours.