A one-step, two-site enzyme immunoassay was developed for measuring human alpha-fetoprotein (AFP) in serum and amniotic fluid using monoclonal antibodies (McAb) by eliminating the high-dose hook effect. Three McAbs that recognize different epitopes were selected among 16 different clones on the basis of epitope mapping, two for immobilization and one for horseradish peroxidase conjugation. This one-step immunoassay system is more convenient and rapid compared to a conventional two-step sandwich immunoassay system. It did not exhibit the hook effect to around 2.7 mg/ml of AFP, which is probably one of the highest concentrations of AFP in the serum. The dose-response curve of the system was linear to 500 mg/ml of AFP and the system could differentiate as low as 1 mg/ml of AFP The intra- and inter-assay variations were in an acceptable range; 95~104% and 97~105% respectively Its correlation with other commercial systems was around 95%.