In a European study of fish death in mixing zones of rivers with different acidities a reliable determination of Al and other elements in gills from freshwater fish was required, and both INAA and ICP-MS were studied as candidate reference methods. INAA requires minimum sample handling with a correspondingly small risk of contamination and no blank value; however, a careful study was needed of both nuclear interference from P and the increased detection limit caused by other major elements in the sample, before reliable results for Al could be ascertained. ICP-MS requires dissolution of the sample with a resulting risk of contamination and a significant reagent blank; while sensitivity was good, the interference from N created problems for sample decomposition in the microwave oven. Our experience with actual samples indicates that both methods suffer from considerable contamination problems, requiring that samples be handled in a clean bench with superpure reagents. Nuclear interference was determined experimentally by irradiating stoichiometric P-compounds with and without a Cd-shield; the observed interference of 1 g Al from 50 mg of P was found to require no correction in almost all cases. The accuracy of results was ascertained by analyzing SRM 1577 Bovine Liver.