In a recent publication in Bioscience Reports "Contaminants in commercial preparations of 'purified' small leucine-rich proteoglycans may distort mechanistic studies", Brown et al. identified by mass spectrometry and immuno-blotting that certain commercial preparations of the small leucine-rich proteoglycans (SLPRs) decorin and biglycan in fact contained a mix of several proteoglycans that also included fibromodulin and aggrecan. The preparations were thus not suitable to study specific activities of decorin or biglycan. Decorin and biglycan are widely studied SLRPs that are considered to have highly multi-functional effects on cells. Decorin is of interest as a transforming growth factor b antagonist and is also finding use in tissue engineering materials. This Commentary discusses Brown et al.'s findings and general issues raised for researchers who work with commercially sourced purified proteoglycans.