Several lines of evidence support the view that premature rupture of the membranes and premature labor are associated with bacterial infection of the amniotic cavity. The standard method of diagnosing intraamniotic infection is amniotic fluid culture, but the results become available only after 72 hours. Methods for more rapid diagnosis of bacterial invasion are therefor needed. We examined leukocyte esterase activity for this purpose. Amniotic fluid obtained from 62 women in preterm labor was cultured for aerobic, anaerobic and mycoplasma species. Leukocyte esterase activity was measured with a Bio-dynamics test strip. The sensitivity of the test was 50.0% and its specificity 93.4%. It was found to be a simple and rapid method for the detection of intraamniotic infection.