We developed a new kit for detecting alpha-fetoprotein (AFP) in leaked amniotic fluid (Eur J Obstet Gynecol Reprod Biol 1995;58:67-72). Later, we developed an improved AFP kit utilizing the same AFP monoclonal antibody. We compared this improved AFP test with the nitrazine test for 137 patients. The nitrazine test correctly diagnosed 62.1% of the cases, but the improved AFP kit diagnosed 98.0% for < 37 weeks of gestation (P < 0.001). The nitrazine test showed a specificity of 58.3%, whereas the AFP kit showed a 100% rate for detecting > or = 37 weeks of gestation (P < 0.01). The reaction time with the AFP kit is 90 s. This study has confirmed a high clinical efficacy of the improved AFP test kit as a method of diagnosis of premature rupture of fetal membranes.