Centers throughout the world have utilized various erythrocyte sedimenting macromolecules to improve the collection of neutrophils from normal donors, but in the United States, high-molecular weight hydroxyethyl starch (HES 4 5 0 / 0 . 7 0 ) is presently the only agent approved by the Food and Drug Administration for use during leukocytapheresis. HES 4 5 0 / 0 . 7 0 , prepared from a waxy species of either maize or sorghum, is hydrolysed by alpha-amylase present in the bloodstream following intravenous administration. The major portion ( 7 0 to 80%) of the injected dose of HES 4 5 0 / 0 . 7 0 is eliminated through the kidneys; a smaller portion (20 to 30%) becomes extravasated, and a minor amount is eliminated through the gastrointestinal tract. If HES 45010.70 is given as a single 500 ml dose, the erythrocyte sedimentation rate (ESR) will be increased and difficulties in blood typing and crossmatching may be encountered. Concentrations of most blood and plasma components will decrease, usually in direct proportion to the amount of HES 45010.70 and other fluids administered. Determinations, both of hepatic and of renal function, will remain within normal limits, although the levels of an alpha-amylase in plasma will become elevated. Serious bleeding complications are normally not observed when the dose of HES 450f0.70 does not exceed 1.0 to 1.5 g/kg body weight. Rarely, anaphylactoid reactions may be encountered. If HES 4 5 0 / 0 . 7 0 is given on more than one occasion, and especially if administered over a relatively short period of time, mild untoward effects related t o expansion of the plasma volume may occur. When HES 4 5 0 / 0 . 7 0 is give chronically, tests of hepatic and renal function remain within normal limits, but certain tests of coagulation may become slightly prolonged. HES 4 5 0 / 0 . 7 0 has an excellent record of safety and is highly recommended for use during leukocytapheresis.