Calcium (Ca)-fortified soymilk has gained popularity in the United States. Tricalcium phosphate (TCP)-fortified soymilk was shown to have a lower Ca bioavailability than cow's milk in men. However, the most popular soymilk in the U.S. is fortified with Ca carbonate (CC) and has not been evaluated. Ca bioavailability from CC-fortified soymilk (CCSM) and TCP-fortified soymilk (TCPSM) was compared with cow's milk in young healthy women using the dual stable isotope technique. In a 3-way crossover design, 20 volunteers (23 +/- 2 y old) consumed 250 mg Ca in cow's milk, CCSM, or TCPSM along with 10 mg 44Ca after an overnight fast. Cow's milk was extrinsically labeled, whereas each fortified soymilk was intrinsically labeled with each chemical salt of 44Ca at the manufacturing facility. Another stable isotope, 43Ca, was injected i.v. 1 h after the complete consumption of cow's milk or soymilk. Fractional Ca absorption was determined from the ratios of 43Ca:42Ca and 44Ca:42Ca by inductively coupled plasma (ICP)-MS in the 24-h urine samples. A mixed linear model (SAS proc mixed) was used to compare the fractional Ca absorption among groups. Fractional Ca absorption in CCSM (0.211 +/- 0.057) did not differ from that of cow's milk (0.217 +/- 0.040), but both were higher (P < 0.05) than that of TCPSM (0.181 +/- 0.039). Our result suggests that calcium absorption is equivalent for CCSM and cow's milk at similar calcium loads.