Elastase is produced and released by polymorphonuclear leukocytes (PNMs) during inflammatory processes. Thus, elastase is assumed to be a sensitive marker of infections similar to the well-established C-reactive protein (CRP). It is deactivated predominantly in tissues by α₁-proteinase inhibitor which forms stable complexes with the elastase molecule (EAPI) that can be detected for several hours. Premature rupture of membranes is often correlated with an early increase in elastase, occurring earlier than the increase in leukocyte count or CRP. Elastase might be a sensitive marker of beginning amnion infection syndrome after premature rupture of membranes. For the present study, plasma EAPI levels of 335 healthy pregnant women as well as 47 healthy nonpregnant pre- and postmenopausal women were analyzed. No significant differences were found in the latter group or in pregnant women until the beginning of labor. Women at the beginning of labor but without rupture of membranes showed a significant increase in plasma EAPI from 97.7 to 338.3 ng/ml (p < 0.001). With opening of the os uteri to more than 2 cm, elastase concentrations decreased to values comparable to those before the beginning of labor (p < 0.001). The use of elastase as a marker for a rupture of membranes or beginning amnion infection syndrome as suggested by a number of studies might need some restriction. As a consequence, serial monitoring of plasma elastase to detect a persisting increase might give more reliable results. The increase in plasma elastase during beginning of labor may be explained by the role of PMNs in the physiology of delivery. However, serial monitoring to detect a persisting increase in plasma EAPI may be more helpful.