Heat treatment is routinely used in the preparation of:therapeutic protein biopharmaceuticals as a means of viral: inactivation. However, in undertaking virucidal :heat-treatments, a balance must be found between the bioprocessing conditions, virus kill, and the maintenance of protein integrity. In this study, we utilize a simple model protein, hen egg-white lysozyme, to investigate the relationship between antiviral bioprocess conditions(protein formulation and temperature) and the extent and type of protein modification. A variety of industrially relevant wet- and dry-heat treatments were undertaken, using formulations that included sucrose as a thermostabilizing excipient. Although there was no evidence of lysozyme aggregation or crosslinking during any of the heat-treatments, using liquid chromatography -electrospray ionization-mass spectroscopy (LC-ESI-MS) and peptide-mapping we show that protein modifications do occur with increasingly harsh heat treatment. Modifications were predominantly found after wet-heat treatment,the major covalent modification of lysozyme under these conditions being glycation of Lys(97), by either glucose-ar fructose derived from hydrolyzed sucrose. The extent of sucrose hydrolysis was itself dependent on both the duration of heat treatment and formulation composition. Advanced glycation end products (AGEs) and:additional unidentified products were also present in protein samples subjected to extended heat treatment. AGEs were; derived primarily from initial glycation by fructose and not glucose. These findings have implications for the improvement of bioprocesses to ensure protein product quality. (C) 2000 John Wiley & Sons, Inc.