Analyzing total peroxidase activity in amniotic fluid is extremely simple, requiring only 1 1/2 minutes of incubation at room temperature and a spectrophotometer. Peroxidase, clearly detectable at as early as 17 weeks' gestation in the amniotic fluid, increases 100-fold by term. The increase follows a predictable rising curve and can be used to estimate the fetal age. Maturation of the fetal salivary gland is most probably the source of rising amniotic fluid peroxidase levels. Samples from complicated pregnancies are lower than normal, do not increase as rapidly, and probably can be bracketed into their own ranges for gestational age evaluation.