A group of 28 uraemic patients on dialysis treatment were given daily supplements of histidine by mouth. Plasma amino-acid concentration, plasma iron, serum transferrin, packed cell volume, and reticulocyte count were all measured before and after two months of histidine supplementation. The treatment raised the plasma histidine concentration and at the same time there was a rise in transferrin and iron levels and packed cell volume. Reticulocyte counts fell after two months of histidine supplementation.