One of the key debates in biomineralisation studies is the extent to which components of the organic matrix become occluded into the crystal lattice during growth. Here, the relationship between protein content and density of calcium oxalate crystals grown in human urine has been investigated in order to determine which fraction of crystal volume is non-mineral. The density of crystals varied from 1.84 to 2.08 g/cm(3) while the protein content ranged from 0.1 to 2.1% (w/w). There was an inverse relationship between measured density and protein content which was qualitatively and quantitatively consistent with predictions based on reasonable densities for the mineral and non-mineral components. The coefficients of the fitted equation suggest that, at 2% protein (w/w), the volume of non-mineral would be 5.0% (v/v). The density values we observed are incompatible with fractional volumes of 20%. The results confirm that the occlusion of a small but possibly significant amount of protein into a crystal lattice is possible, but cast doubt on the hypothesis that protein acts as a major intracrystalline ultrastructural element. Moreover, the methodology developed for this study offers a simple and robust method for interrogating organic/inorganic associations in a range of biological and medical systems. (C) 2003 Elsevier Science (USA). All rights reserved.