Monoclonal antibodies to basic fetoprotein (BFP) were produced by hybridoma technology and the immunoreactivity of monoclonal anti-BFP to cancer tissues was investigated by an immunohistological technique. The investigation indicated that one type of monoclonal anti-BFP had a higher specific immunoreactivity to cancer. With this monoclonal anti-BFP, an enzyme immunoassay (MAb-EIA) was developed based on a competitive binding technique using biotin-labeled BFP and peroxidase-conjugated avidin. By the MAb-EIA and an enzyme immunoassay developed with conventional polyclonal anti-BFP (PAb-EIA), BFP was determined in sera from 95 normal donors, 238 patients with benign disease and 580 patients with cancer. Cut-off values of 90 ng/ml in MAb-EIA and 80 ng/ml in PAb-EIA were used. On comparing MAb-EIA with PAb-EIA, we found similar positive incidence for normal donors (MAb-EIA 0% PAb-EIA 2.1%) and patients with benign disease (MAb-EIA 26.0%, PAb-EIA 24.4%), but higher positive results in MAb-EIA (46.7%) for patients with cancer compared with those in PAb-EIA (33.8%).