The urine of normal persons, as well as that of stone formers, contains inhibitors to crystallization of calcium oxalate. To ascertain differences, if any, between these 2 groups we measured nucleation rate, growth rate and total mass produced in an analytic system with 5% urine added to 95% synthetic urine that did not contain large organic molecules. We also observed the effects of addition of uromucoid to the synthetic solution. Normal urine and stone-forming urine contained significant inhibitors to total mass of calcium oxalate dihydrate crystals precipitated but total crystal mass was not significantly different between the 2 groups. Growth rate was significantly less in experiments with the urine of stone formers. The other major difference found in stone-forming urine was significant enhancement of nucleation rate when compared to normal urine. Since addition of uromucoid produced these same growth inhibition and nucleation enhancement effects but did not affect total crystal mass, we hypothesize that uromucoid has an important part in producing the differences noted between normal and stone-forming urine.