Two hybridoma cell lines 1B5 and 2F1 capable of secreting specific monoclonal antibodies (McAbs) against aflatoxin B1 (AFT B1) were obtained. BALB/c mice were immunized by intrasplenic injection of AFT B1-human serum albumin conjugate (AFT B1-HSA) containing 9 AFT B1 residue per molecule. Spleen cells of BALB/c mouse with high serum antibody titer were fused with SP 2/0 myeloma cells in the presence of polyethylene glycol. Hybridoma cell lines were selected by using an indirect ELISA with AFT B1-keyhole limpet haemocyanin as coating antigen and grown as ascite tumour cells in BALB/c mice which previously had received an intraperitoneal injection with Freund's incomplete adjuvant. The McAbs were found to have strong reaction with AFT B1-bovine serum albumin conjugate (AFT B1-BSA) and no reaction with HSA or BSA. The reactivity of the McAbs with AFT analogs was determined by an indirect inhibition ELISA and the concentrations (ng/ml) required to inhibit 50 binding of McAb to AFT B1-BSA conjugate solid phase were AFT B1 4.0ng/ml, B2 36.9ng/ml, G1 23.3ng/ml and G2 403.3ng/ml for 1B5 McAb, and AFT B1 2.4ng/ml, B2 2.6ng/ml, G1 2.8ng/ml and G2 4.7ng/ml for 2F1 McAb.