1. Chromium sesquioxide (Cr2O3; 1.5 g/day) and [51Cr]chromic chloride [51CrCl3 (0.3 muCi/day)] were compared as continuously administered non-absorbed markers for the correction of faecal recoveries in 14 calcium balance studies each lasting at least 18 days. 2. The mean recoveries of each, 98.4% for Cr2O3 and 101.9% for 51CrCl3, were not significantly different from 100%. 3. The two markers reduced the uncertainity in a typical 3 x 6 day calcium balance study to a similar extent (SD = 1.4 mmol/day for Cr2O3 and SD = 1.5 mmol/day for 51CrCl3. 4. 51CrCl3 is a convenient and satisfactorily alternative to Cr2O3 when the laboratory hazards associated with estimating the latter cannot easily be eliminated.