Fresh samples of human urine adjusted to pH 6.8 were rapidly evaporated at 37 degrees C to 1,250 mosmol/kg. The calcium phosphate precipitated was washed and the calcium measured by flame atomic absorption spectrophotometer. This procedure was found to be reasonably reproducible. When macromolecules were first removed by ultrafiltration the precipitated calcium was reduced by 76.9%. Addition of human Tamm-Horsfall mucoprotein (T-H) to the urine ultrafiltrates prior to evaporation largely but not completely restored the precipitated calcium. Clumping of crystals was studied quantitatively by passage through a nylon mesh. Mean retention on the mesh was strikingly reduced after ultrafiltration and increased by addition of T-H to the ultrafiltrates. These findings support the view that T-H triggers calcium phosphate crystal formation and clumping in whole urine.