Metabolic activation of the hepatocarcinogenic mycotoxin aflatoxin B(1) (AFB(1)) results in the covalent attachment of AFB(1) to serum albumin. Digestion of adducted albumin releases AFB(1)-lysine, a biomarker of exposure status. AF-albumin adducts have been most frequently measured in precipitated serum albumin using an immunoassay (ELISA); however, a sensitive and specific isotope dilution mass spectrometric (IDMS) assay for measurement of AFB(1)-lysine in serum has recently been developed. The ELISA and IDMS methods were compared using 20 human sera collected in Guinea, West Africa, where AF exposure is endemic. Measurement of AFB(1)-lysine adduct concentrations by IDMS in serum and albumin precipitated from the same sample revealed that precipitation has no effect on the measured adduct levels. The concentration of AF-albumin adducts measured by ELISA and AFB(1)-lysine measured by IDMS in 2 mg of albumin were well correlated (R = 0.88, P < 0.0001); however, AF-albumin adduct concentrations measured by ELISA were on average 2.6-fold greater than those of the AFB(1)-lysine adduct. Although these data suggest that the ELISA is measuring other AF adducts in addition to AFB(1)-lysine, these biomarkers are comparable in their ability to assess AF exposure at AF-albumin concentrations > or =3 pg AFB(1)-lysine equivalents/mg albumin. Identification of other adducts may clarify the mechanistic basis for using AF-protein biomarkers to assess exposure status in future epidemiologic studies of liver cancer.