Timed urinary collections from 8 normal (Nl) persons of 11 stone forming (SF) patients were passed through ultrafiltration apparatus to remove macromolecules in the ranges 1000-30,000 d, 30,000-50,000 d, and over 50,000 d. No macromolecules could be recovered from either group in the 30,000-50,000 d range, and no low molecular weight macromolecules (LMWMM) (less than 30,000 d) were recovered from stone forming urines. Significant amounts of LMWMM (mean 105.8 +/- 17.63 mg./l.) were recovered from normal urine, but these extracts had no effects on calcium oxalate dihydrate (COD) nucleation (Bo) or linear growth (G) rates in a continuous crystallization (MSMPR) system. Urines from SF contained nearly twice the concentration of high molecular weight macromolecules (HMWMM) when compared to Nl urines. SF HMWMM differed from Nl in immunoelectrophoresis separation by absence of a dense band that was present in Nl extracts. This band reappeared in SF extract after boiling. Comparison of effects of addition of SF or Nl HMWMM to the COD-MSMPR crystallization system revealed no major quantitative differences in Bo or G, but SF HMWMM had a remarkable stabilizing effect on total mass (MT) of COD crystals produced. This effect was confirmed by analysis of oxalate residual supersaturation after crystallization. We conclude that SF excrete higher concentrations of HMWMM and almost no LMWMM when compared to normals. This higher concentration of HMWMM must contribute to increased Bo and decreased G noted in SF urine additive experiments previously reported. The mechanism of rapid removal of oxalate (i,e, stabilization) noted in experiments with SF HMWMM is not obvious at this time.