Macromolecules were removed from whole urine samples by sequential ultrafiltration using molecular weight cut-offs of 300,000, 100,000 and 10,000 dalton. The whole urine samples and ultrafiltrates were evaporated to 1,250 mosm/kg at pH 5.3 and 6.8 to study calcium oxalate and calcium phosphate crystal formation, respectively. While all the ultrafiltrates removed Tamm-Horsfall proteins and most of the crystal formation, there was some 10% of the crystal-promoting powers in the macromolecules in the fraction between molecular weight 100,000 and 300,000 dalton. This demonstrates the existence of a second crystal-promoting substance in the macromolecules of whole urine.