It is generally accepted, that lipid peroxidation plays a pathogenic role in atherosclerosis. Furthermore, recent studies indicate that antibodies directed against oxidative modifications of Low Density Lipoprotein (oLAb) contribute to atherosclerotic processes and may have some function in other disorders. These antibodies have been determined predominantly in humans, because assays for oLAb measurement use species specific anti IgG conjugates. From such assay designs it is not possible to get directly comparable data from various animal species. Main advantages of comparable data between animal species are that results of animal experiments can be interpreted using human calibrators and that results of immunisations and production of monoclonal antibodies are directly comparable not only within, but also between animal species. The aim of this study was to find a modification for ELISAs for oLAb determination, which allows to measure sera of various animal species simultaneously. Microtitration plates were coated with oxidised LDL and blocked with bovine serum albumine. Human and animal sera were then pipetted into the plate in logarithmic serial dilutions and incubated for 2 h at 37 degrees C. After washing, a protein A horse-radish peroxidase conjugate (Biomakor, Israel) was added to each well in a dilution of 1:20,000. The incubation conditions had to be optimized to achieve reliable results. After another washing step, the assay was developed with TMB. Absorptions were read at 450 nm in a microplate photometer. Following the manufacturers incubation instructions, which recommended a duration of 1 h at room temperature, the system did not work optimally. No binding of protein A to IgG molecules bound to oxidised LDL could be observed, if the system was incubated at 37 degrees C. In our hands, best results were achieved for several animal species, if the conjugate was incubated for two hours at 2-4 degrees C in a refrigerator. Under these conditions, assay sensitivity was the same as in the standard method, which uses anti-species IgG conjugates. The protein A modification of oLAb allows direct reading of animal oLAb titres from human calibrators. With this method, results of animal experiments can be interpreted on the basis of the situation in humans. Preliminary results obtained show that immunisation experiments with oxidised LDL give serum titres in animals, which are in the same order of magnitude as human sera with high oLAb concentrations. The results of this study, in accordance with findings of other authors, give further indications that atherosclerotic processes are influenced by the specific immune system.