Twenty-five patients with end-stage renal disease, nine of whom were receiving pharmacologic doses of vitamin D, and seventeen patients with primary hyperparathyroidism underwent bone biopsy following a three-day course of tetracycline administration. The mean width of the fluorescent tetracycline bands were significantly greater in the bones of patients with uremia than in those with primary hyperparathyroidism. This difference was due to wide labels present in the patients with uremia who had not been treated with vitamin D, as no differences existed in mean label widths of patients with uremia who had received this compound and the patients with primary hyperparathyroidism. Comparison of the maximum label widths distinguished not only primary hyperparathyroid patients from those with uremia, but uremic patients who had received vitamin D from those who had not been so treated. Quantitative microscopy of standard, nonfluorescent histologic features failed to make this latter distinction. These data are consistent with the presence of a wide zone of instantaneously fluorescing material in uremic bone following tetracycline administration, which does not relate to bone apposition occurring during antibiotic administration. This phenomenon probably represents a delay in mineral maturation which is normalized by vitamin D. Furthermore, it is apparent that the use of a continuously administered (single) tetracycline label will result in an overestimation of bone formation rates, particularly in osteomalacic states.