Age-related changes in the primary structure of alcohol dehydrogenase (ADH) from three different strains (Adh-F, Adh-D and Adh-S) of Drosophila melanogaster have been investigated by tryptic peptide analysis. The procedure involves isolation of 14C-labelled ADH, immunoprecipitation and HPLC separation of tryptic peptides. This method is rapid, reproducible and sensitive. By using this procedure it is demonstrated that there are no significant differences between the ADH tryptic peptide profiles of young (10-day-old) and old (50-day-old) flies. The usefulness of this procedure for screening mutant and variant enzymes is discussed.