1. The urinary F1 activation peptide of prothrombin is the predominant protein incorporated into calcium oxalate crystals precipitated from human urine. The aim of this study was to examine the effect of pure urinary prothrombin F1 on calcium oxalate crystallization in human urine. 2. Urinary prothrombin F1 was purified from demineralized calcium oxalate crystals precipitated from human urine, and its effects on calcium oxalate crystallization induced by addition of an oxalate load were tested in undiluted, ultrafiltered urine from healthy men, at final concentrations of 0 to 10 mg/l. 3. Urinary prothrombin F1 did not affect the amount of oxalate required to induce crystallization, but the volume of material deposited increased in proportion to increasing concentrations of urinary prothrombin F1. However, the mean particle size decreased in reverse order: this was confirmed by scanning electron microscopy, which showed it to be the result of a reduction in crystal aggregation rather than in the size of individual crystals. Analysis of 14C-oxalate data revealed a dose-dependent decrease in calcium oxalate deposition with an increase in urinary prothrombin F1 concentration, indicating that the increase in particle volume recorded by the Coulter Counter resulted from inclusion of urinary prothrombin F1 into the crystalline architecture, rather than increased deposition of calcium oxalate. 4. It was concluded that urinary prothrombin F1 may be an important macromolecular determinant of stone formation.