Prolidase (EC: 188.8.131.52) catalyses the hydrolysis of the peptide bond involving the imino nitrogen of proline or hydroxyproline. Because of the high proportion of imino acids in collagen, this enzyme plays an important role in its degradation. Since collagen turnover rate is expected to be high during fetal growth, the level of prolidase activity may reflect the degree of fetal maturation. In this study, amniotic fluid prolidase I activity was measured in term and preterm pregnancies. Lecithin concentration, which has been widely used for predicting fetal lung maturity, was also measured. Prolidase I activity was positively correlated with lecithin levels (n = 30; r = 0.42; p < 0.02), and also with birth weight of the babies (n = 30; r = 0.52; p < 0.01) in the term-mature group. Dysmature babies had significantly lower prolidase I activity in the amniotic fluid which was thought to be indicative of growth retardation.