The effect of lactose on intestinal calcium absorption was studied in 12 normal-lactase subjects and 7 lactase-deficient subjects. Calcium absorption was compared by using the double isotope technique with or without the presence of an oral load of 50 g of lactose in each subject. The kinetic parameters were measured by the deconvolution method. In the normal lactase group, the lactose prolonged the duration of absorption at a maximum rate and therefore increased the total fractional calcium absorption from 0.224 +/- 0.039 to 0.356 +/- 0.076 (p less than 0.001). In the lactase-deficient subjects, the lactose decreased the total absorption from 0.255 +/- 0.039 to 0.209 +/- 0.032 (p less than 0.005); this effect resulted in a decrease of the maximum absorption rate. There was no significant difference between the two groups when calcium was ingested alone. These results indicate that the effect of lactose on calcium absorption is dependent on intestinal lactase activity.