The formation of calcium oxalate (CaOx) crystals, their growth and above all their agglomeration tendency are considered to be decisive factors in the renal stone formation. The aim of this study was to verify whether the formation in vitro of crystalline macroaggregates, induced by increasing loads of oxalate, is different in normal subjects as opposed to CaOx stone formers, free from urinary metabolic abnormalities. The possible interference of the urinary micromolecular environment on the activities of macromolecules with molecular weight greater than 10,000 Daltons was also evaluated. Formation of crystalline macroaggregates took place in controls at a relative CaOx supersaturation equivalent to 32.6 +/- 12.9 and in stone formers at 34 +/- 12.3 (NS); the CaOx RS aggregation--CaOx RS nucleation delta was 11.4 +/- 12.2 in controls and 15.8 +/- 13.9 in stone formers (NS). Leaving aside the stone disease variable, the urine characterized by high baseline CaOx saturation, high ionic strength and high levels of citrate, magnesium and calcium, seemed particularly predisposed to macroaggregates formation; the opposite was true for urine characterized by a totally different micromolecular environment. A very close relationship was found between the CaOx RS aggregation--CaOx RS nucleation delta in urine filtered at 0.22 mu and in metastable solutions containing macromolecules in similar concentrations to those found in the native urine (r = 0.74, P < 0.0001). In conclusion the formation of crystalline macroaggregates in vitro does not present differences in normal subjects compared to CaOx stone formers free from urinary metabolic abnormalities. The urinary macromolecules seem to participate in the formation of the crystalline macroaggregates and their action seems to be affected by the urinary micromolecular environment and, even when they are re-suspended in a completely different medium, their action seems to be preserved intact.