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Calcium bioavailability was defined as either retention of 45Ca in tibias (Experiment 1) or retention of 47Ca in carcasses (Experiment 2). In Experiment 1, rats (age 21, 40 or 100 d) were fed purified meals extrinsically labeled with 45Ca. The meals contained either 0.5% Ca (control) or 1% Ca [control supplemented with CaCO3, calcium citrate-malate (CCM), milk or cheese] and either no lactose or 20% lactose. Lactose increased Ca bioavailability (P less than 0.05) from the control and milk meals in all age groups. Increases from CCM and CaCO3 were significantly only in the 21-d-old group. Lactose did not affect bioavailability from cheese. In Experiment 2, suckling rats (age 7, 12 or 17 d) were gavaged with 47Ca-labeled milk (fluid skim or lactose-hydrolyzed fluid skim) or an aqueous CaCl2-casein mixture (containing either no sugar, glucose + galactose, or lactose). Bioavailability from milk was higher than from lactose-hydrolyzed milk in all age groups. Lactose and glucose + galactose increased bioavailability over the sugar-free CaCl2-casein mixture in all age groups. Data from these experiments show that lactose enhances Ca bioavailability at several stages of development and the effect is not markedly diminished by high Ca diets. Lactose increases Ca bioavailability from a variety of sources but the magnitude of the effect may vary among sources.