The total excretion of inhibitors of in vitro calcification was measured (in inhibiting units per day) in 24-hour urine samples of 11 control subjects and 20 patients with renal calculi. A semiquantitative method incorporating the rachitic rat cartilage technique was used. In both groups there was a significant positive correlation between the number of inhibiting units per day and the daily urine volume. The mean number of inhibiting units per day was significantly (P smaller than 0.05) higher in the stone patients than in the controls. However, the stone-formers had significantly larger (P smaller than 0.01) 24-hour urine volumes. When corrections were made for urine volume there was no significant difference between the two groups. These data suggest that the underlying abnormality responsible for renal stone formation is not a persistent decrease in the total concentration of urinary inhibitors of calcification.