We examined the effect of ingested casein phosphopeptides (CPP) on intestinal calcium (Ca) absorption and determined the minimum effective dose for enhancement of Ca absorption under conditions of marginal dietary Ca levels. Male Sprague-Dawley rats, divided upon weaning into five groups, were fed a control diet (isolated soyprotein, ISP; 20%) or a CPP diet (ISP + CPP; 20%, CPP/Ca: 0.1, 0.2, 0.35 and 1.0) for 4 weeks. All diets contained the same amounts of Ca (0.35%) and phosphorus (0.70%). The apparent Ca absorption, the retention of Ca, and the luminal soluble Ca content in the small intestine as determined at 4 weeks in the rats fed CPP diet with a weight ratio of CPP/Ca of more than 0.2 were significantly (p < 0.05) higher than those in the rats fed control diet. The wet weight, length and Ca content of the femur were not significantly different among the groups. These results indicate that the minimum effective dose of CPP for enhancement of Ca absorption is 0.7 g/kg or a weight ratio of CPP/Ca of 0.2 in the ISP diet and that CPP supplementation has the effect of significantly increasing Ca absorption at least under conditions of marginal dietary Ca levels.