This work demonstrates that pressure-induced partially unfolded states play a very important role in the aggregation of proteins. The high-pressure unfolding of horse heart metmyoglobin results in an intermediate form that shows a strong tendency to aggregate after pressure release. These aggregates are similar to those that are usually observed upon temperature denaturation, Infrared spectra in the amide I region indicate the formation of an intermolecular antiparallel beta-sheet stabilized by hydrogen bonding. The formation of the aggregates is temperature-dependent. Below 30 degrees C, no aggregation is taking place as seen from the infrared spectra. At 45 and 60 degrees C, two types of aggregates are formed: one that can be dissociated by moderate pressures and one that is pressure-insensitive. When precompressed at 5 degrees C, temperature-induced aggregation takes place at lower temperature (38 degrees C) than without pressure pretreatment (74 degrees C).