The estimation of the degree of saturation of brushite and of calcium oxalate in urine of patients with disorders of Ca metabolism was an effective tool in evaluating the patient''s propensity to form renal stones. The methods used in 3 different laboratories were compared and evaluated in 27 urine samples. Relative saturation ratio (ratio of activity product and thermodynamic solubility product of brushite) and activity product ratio (ratio of activity product of original sample and that obtained after incubation of sample with synthetic brushite) were determined. Similar studies were performed for calcium oxalate. The values for relative saturation ratio of brushite of calcium oxalate differed widely among the 3 techniques. Nearly identical values for activity product ratio of brushite were obtained. The activity product ratios of calcium oxalate, obtained by different techniques, closely approximated each other except in samples containing excessive amounts of Ca (> 5 mM) or oxalate (> 0.5 mM). The values for the relative saturation ratios for both brushite and calcium oxalate were usually higher than the corresponding values for activity product ratio. The use of activity product ratio provided a more simple and reliable estimate of the urinary state of saturation than did the relative saturation ratio.