Chromatographic separation of urine showed inhibition of calcium oxalate (CaOx)-crystallization among substances with both large and small molecular weights. Ultrafiltration showed that approximately 80 per cent of the inhibiting activity, as determined in 2 per cent urine, originated from substances with a molecular weight above 1,000. Dialysed urine was diluted to 7.5 mmol of creatinine per 1 and supersaturated with respect to CaOx. The rate of crystallization in these samples was slower in normal subjects than in stone formers (p less than 0.05). The inhibiting activity in diluted urine from the two groups did not differ and neither did the concentration of alcian blue precipitable polyanions. From measurements in diluted urine it was apparent that inhibition was demonstrable with a urine concentration as low as 0.3 per cent.