Centrifuged and filtered urine is often used to evalu ate in vitro the crystallization processes ot calcium oxalate (CaOx), but even such simple manipulations can alter the composition of the urine, as regards its protein and Iipid 000centrations. In urine samples tak en from 17 normal male adults, we evaluated CaOx crystallization by simultaneously using three different types of urine: untreated (U), centrifuged at 2000 rpm (800 g) and filtered at 0.22 ttm (CF), and centrituged flltered and ultrafiltered at 10000 Da (CFU). The addi tion ot 1.2 mmol/I of oxalate to each type of urine produced notably different results. The total amount of CaOx crystals (expressed as calcium oxalate dihy drate crystals (COD)+oxalate monohydrate crystals (COM) area/total area x100) was on average 13.2% in U urine, 70.7% in CF urine and 11.1% in CFU urine (CF>U and CFU, U=CFU); the relative prevalence of COD and COM (expresserj as COD area/COM area) was on average 71.4 in U urine, 0.0026 in CF urine and 5.5 in CFU urine (U>CF and CFU, CFU>Cfl; the diameter of COD (expressed in microns) was on aver age 15.2 in U urine, 3.7 in CF urine and 24.3 in CFU urine (CFU>U and CF, U>CF); the diarneter of COM (expressed in microns) was on average 5.2 in U urine, 2.6 in CF urine and 8.9 in CFU urine (CFU>U and CF, U>CF); the total amount of CaOx aggregates (expressed as CaOxAgg area/total areaxlOO) was average 8.5% in U urine, 22.1% in CF urine and 2.9% in CFU urine (CF>U and CFU, U>CF). We conclude that CaOx crystallization processes in manipulated urine are extremely different, probably due to chang es in macromolecular compounds.