Direct and indirect competitive enzyme-linked immunosorbent assays were optimized for the determination of aflatoxin B1 in groundnut utilizing a specific monoclonal antibody developed at the University of Strathclyde, UK. The monoclonal antibody was conjugated to horseradish peroxidase (HRP) for direct competitive assay, while a commercially available goat-antimouse IgG-HRP conjugate was employed for indirect competitive ELISA. Both ELISAs detected aflatoxin B1 as low as 20 pg/well. Methanol-water-KCl (70 + 30 v/v, 0.5 %) extracts of groundnut were assayed by ELISA after diluting 1: 10 with PBS-Tween buffer or subjected to simple cleanup for 5:1 concentration prior to assay. The mean recoveries from groundnut spiked with 10 to 200/ig/kg of pure aflatoxin B1 were >90% in either ELISA, but the toxin recoveries at concentrations of 1-5μg/kg were only 65-67 % when subjected to cleanup and concentration before assay. The mean within-assay, inter-assay, and sub-sample coefficients of variation by ELISA of aflatoxin B1 in naturally contaminated groundnuts were, respectively, 8.9%, 11.1%, and 7.9% for direct competitive assay and 4.6%, 11.2%, and 8% for indirect competitive assay. Both ELISA methods are useful for routine analysis of aflatoxin B1 in groundnuts.