The objective of the present study was to investigate the validity of using holmium as a faecal marker in a copper absorption study using a highly enriched (65)Cu stable isotope label. Ten volunteers (nine female, one male) aged 24-55 years were recruited from the Norwich area to take part in a free-living study. The study was conducted in the Human Nutrition Unit at the Institute of Food Research, Norwich, U.K., and involved feeding breakfast test meals containing foods labelled intrinsically or extrinsically with a highly enriched (65)Cu stable isotope label. A 1 mg oral dose of the rare earth element holmium was administered to volunteers simultaneously with an oral dose of highly enriched (65)Cu stable isotope as a label in the breakfast test meal. Complete faecal collections were made for 10 days after dosing, and the mean percentage holmium recovery (+/- S.D.) was 105 +/- 15%. After correcting for re-excreted copper label, the labelled copper and holmium had similar excretory profiles. It was concluded that holmium is a valid faecal marker in adult human copper absorption studies using stable isotopes.