Background: In the cellular environment, macromolecules occupy about 30% of a cell's volume. In this crowded environment, proteins behave very differently than in dilute solution where scientists typically study the properties of proteins. For this reason, recent studies have investigated proteins in cell-like crowded conditions so as to understand if this changes their properties. The present study was performed to examine if molecular crowding impedes the protein unfolding process that is known to occur upon the application of high pressure. Methods: Crowding of staphylococcal nuclease (SNase) was induced by dissolving low concentrations of SNase in high concentrations of crowding agents (16 wt.% or 25 wt.% PEG 3000 or 16 wt.% Dextran T10). SNase unfolding was then monitored via tryptophan fluorescence as pressure was applied. Results: Fluorescence spectra can be decomposed into the sum of two components indicative, respectively, of native and unfolded states, and the center of spectral mass was then used as a measure of the degree of protein unfolding. It was found that SNase unfolding as a function of pressure was impeded in crowded solutions. These results suggest that crowded environments, such as those found in the cellular cytoplasm, may also impede high-pressure protein unfolding in cells. General significance: This is the first report on the effect of crowding on the pressure-induced unfolding of a protein (staphylococcal nuclease) monitored via tryptophan fluorescence. © 2012 Elsevier B.V. All rights reserved.