1. The plasma disappearance of 3H-labelled 25-hydroxyvitamin D3 (25(OH)D3) was studied in healthy volunteers on normal and high-fibre diets, using 3H-labelled tracer doses given intravenously. 2. The mean (+/- SEM) plasma half-life in the high-fibre-diet group was 19.2 +/- 1.7 d, which was significantly shorter than in the group on normal diets (27.5 +/- 2.1 d, P less than 0.01). 3. This finding suggests that a high-fibre diet leads to enhanced elimination of 25(OH)D3 by an action within the intestinal lumen. This may involve interference with an enterohepatic circulation of the metabolite, perhaps by binding of 25(OH)D3 to dietary fibre. 4. The reduced plasma half-life of 3H-labelled 25(OH)D3 associated with a high-fibre diet may explain the development of vitamin D deficiency in Asian immigrants with normal exposure to u.v. light.