The aim of this study was to compare directly, in the absence of interfering contaminants, the inhibitory effects of Tamm-Horsfall glycoprotein (THG), human serum albumin (HSA), alpha1-microglobulin and prothrombin fragment 1 (PTF1) on calcium oxalate crystallization. These proteins have been detected in urinary calculi, and with the exception of THG in calcium oxalate crystals generated from undiluted human urine. THG was isolated from the urine of healthy men, while PTF1 was purified from Prothrombinex-HT, a human blood concentrate; HSA and alpha1-microglobulin were obtained from commercial sources. The effects of these proteins were determined, separately, at the same final concentration (32 nM) on calcium oxalate crystallization in a seeded, inorganic reaction system, using Coulter Counter and [14C]oxalate analysis. Analysis of [14C]oxalate data showed that THG, HSA and alpha1-microglobulin had no measurable effect on deposition of calcium oxalate. However, PTF1 significantly inhibited mineral deposition by 19.6%. The average size of the particles precipitated was reduced from the control value of 8.6 microm to 7.3, 5.9, 5.6 and 4.0 microm in the presence of alpha1-microglobulin, HSA, THG and PTF1 respectively. These findings were confirmed by scanning electron microscopy, which also revealed that the smaller particles deposited in the presence of the proteins resulted from reduced crystal aggregation rather than a decrease in the size of the individual crystals. It was concluded that, on a molar basis, PTF1 is a more potent inhibitor of calcium oxalate crystal aggregation than THG, HSA and alpha1-microglobulin. Moreover, unlike those proteins it significantly inhibits the deposition of calcium oxalate. These findings have implications for the putative role of urinary proteins in the formation of calcium oxalate stones.