In vitro tests of calcium (Ca) availability and the oral Ca load test were conducted on eight brands of calcium carbonate (CaCO3) tablets (products A-H) each providing 500 mg Ca. Data were collected over three experiments with nine to 11 healthy premenopausal women testing two to four products. Subjects followed a low Ca diet (less than 10 mmol/day). On test mornings, fasting subjects collected baseline urine for 2 hours (−2 to 0 hours), then ingested the tablet with water. Urine was collected from 0 to 2 hours and 2 to 4 hours; for products E-H, urine was collected for an additional 2 hours (4 to 6 hours). Blood was sampled at hours 0, 4 and 6 during testing of products E and F. Three in vitro tests were run: the United States Pharmacopeia (USP) dissolution test, USP disintegration test, and a disintegration test using vinegar. Ca excretion rose significantly at hours 2 to 4 (p less than 0.05) compared to baseline for seven products. Ca excretion either fell or remained constant between hours 4 and 6. Serum Ca rose and serum parathyroid hormone fell at hour 4, compared to fasting values, suggesting that 4 hours represents peak response time. In vivo availability, measured as the incremental increase in Ca excretion (mmol/mmol creatinine) in hours 2 to 4 compared to baseline, did not correlate significantly with results of the USP dissolution test but did with results of either the USP disintegration test or the vinegar test.(ABSTRACT TRUNCATED AT 250 WORDS)