We evaluated the bioavailability of two types of calcium from milk in two experiments. One was a micellar calcium phosphate-phosphopeptide (MCP-PP) complex in which the chemical form was similar to the original form of milk, and the other was a commercial whey calcium in which the chemical form was different from that of milk. In experiment 1, the calcium absorption, bone mineral density, and bone strength were examined when growing female rats were fed either MCP-PP complex or whey calcium as the sole source of calcium for 46 d. In experiment 2, the calcium solubility in the small intestine was measured when female rats were meal-fed either MCP-PP complex or whey calcium. The apparent calcium absorption rate in both groups decreased time-dependently during the experimental period, but the time-dependent change in the apparent calcium absorption rate was statistically different. It decreased more slowly in rats fed the MCP-PP diet than in rats fed the whey calcium diet. The bone mineral density of the femur in rats fed the MCP-PP diet was significantly higher than that of the rats fed the whey calcium diet. The bone strength (breaking force and energy) of the femur in rats fed the MCP-PP diet was higher than in the rats fed the whey calcium diet. The amount of soluble calcium in the small intestinal contents in rats at 2.5 h after ingestion of the MCP-PP diet was approximately three times higher than that found in rats fed the whey calcium diet. These results indicate that the calcium bioavailability of MCP-PP complex is higher than that of whey calcium, and this difference is due in part to the solubility in the intestine.