1. The uptake of either orally administered [3H]vitamin D3 or 25-[3H]hydroxy-vitamin D3 into the chylomicron fraction of plasma was studied in 12 healthy male subjects.
2. The amount of [3H]vitamin D3 in the chylomicron fraction expressed as a percentage of the total plasma radioactivity was higher than that of 25-[3H]hydroxy-vitamin D3.
3. It is suggested that the mechanism of intestinal absorption of vitamin D and 25-hydroxy-vitamin D may differ in man, the absorption of 25-hydroxy-vitamin D possibly being less dependent on bile acids.