Brushite (CaHPO4ยท2H2O) was considered to govern the formation of renal calculus of calcium phosphate origin. The degree of saturation of urine with respect to this phase was therefore calculated. This value was obtained from the ratio of the activity product of Ca++ and HPO4m (Ksp) before and after incubation of urine with brushite. The errors in the calculation of Ksp were largely eliminated by this procedure.
