Technical Abstract: With the use of stable isotopes, this study aimed to compare the bioavailability of active absorbable algal calcium (AAACa), obtained from oyster shell powder heated to a high temperature, with an additional heated seaweed component (Heated Algal Ingredient, HAI), with that of calcium carbonate. In 10 postmenopausal women, volunteers aged 59 to 77 years (mean +/- S.D., 67 +/- 5.3), the fractional calcium absorption of AAACa and CaCO3 was measured by a dual stable isotope method. 44Ca-enriched CaCO3 and AAACa were administered in all subjects one month apart. After a fixed-menu breakfast and pre-test urine collection (Urine 0), 42Ca-enriched CaCl2 was intravenously injected, followed by oral administration of 44Ca-enriched CaCO3 without carrier 15 minutes later, and complete urine collection for the next 24 hours (Urine 24). The fractional calcium absorption was calculated as the ratio of Augmentation of 44Ca from Urine 0 to Urine 24/ augmentation of 42Ca from Urine 0 to Urine 24. Differences and changes of 44Ca and 42Ca were corrected by comparing each with 43Ca. Fractional absorption of AAACa (mean +/- S.D., 23.1 +/- 6.4), was distinctly and significantly higher than that of CaCO3 (14.7 +/- 6.4; p = 0.0060 by paired test). The mean fractional absorption was approximately 1.57-times higher for AAACa than for CaCO3. The serum 25(OH) vitamin D level was low (mean +/- S.D., 14.2 +/- 4.95 ng/ml), as is common in this age group in Japan. Among the parameters of the bone and mineral metabolism measured, none displayed a significant correlation with the fractional absorption of CaCO3 and AAACa. Higher fractional absorption of AAACa compared with CaCO3 supports previous reports on the more beneficial effect of AAACa than CaCO3 for osteoporosis.