Two enzyme immunometric assays suitable for measuring native and denatured beta-lactoglobulin (BLg) have been developed. The assays were performed in 96-well microtitre plates and were based on the use of pairs of monoclonal antibodies specific to either the native form or the reduced and carboxymethylated form of BLg (RCM-BLg). Detection limits of 30 and 200 pg/ml were obtained for the native BLg and the RCM-BLg assay, respectively, with very low or negligible cross-reactivity of the other milk proteins and tryptic fragments of BLg. The validity of the assays in different media such as cow's milk and cow's milk products, saline buffer or serum was supported by recovery experiments. The assays were first applied to the determination of BLg and RCM-BLg in PBS and in raw skimmed milk. The ability of the RCM-BLg assay to detect heat-denatured BLg was confirmed by a kinetic study of BLg heat-denaturation in the two media. During heat treatment, the decrease in the concentration of native BLg was associated with an increase in denatured BLg specifically detected by the RCM-BLg assay. By selecting an appropriate monoclonal antibody which failed to recognize caprine BLg, we were able to establish a modified sandwich immunoassay permitting very sensitive detection of cow's milk in goat's milk.