In normal heparinized blood, the retention of platelets in glass bead columns was low in the first 1 or 2 ml, increasing to more than 80% by the 4th or 5th ml. Prior flushing of the columns with platelet-poor plasma or saline lowered retention in all 5 ml. Additional studies were carried out with a two-stage procedure in which a sample of blood (A) was pumped through a column, immediately flushed out with saline or plasma, and followed by a second blood sample (B). When as little as 1 ml of blood A preceded the flushing solution, retention was very high in all 5 ml of the subsequent blood B. This enhancement of retention in B occurred, providing blood A contained platelets (other than thrombasthenic), fibrinogen, and adequate divalent cations. Enhancement did not require von Willebrand factor (vWF) in A, nor was ADP necessary, since enhancement occurred even when heparinized blood as A contained prostaglendin E1 (PGE1) or creatine phosphokinase with creatine phosphate (CPK-CP). However, the presence of PGE1 or CPK-CP in the plasma used to flush the columns prevented the enhancement of retention in the first milliliter of B. Retention in the first milliliter of B (following normal blood as A and saline or normal plasma for flushing) was high when B was afibrinogenemic, moderately high when B contained PGE1 or CPK-CP, and low in thrombasthenic, EDTA, or vWF-deficient blood. Retention declined in subsequent milliliters of PGE1 or CPK-CP blood and remained low in thrombasthenic, vWF-deficient, or EDTA blood. Our findings suggest that (1) the platelets in A adhere to glass; this adhesion requires fibrinogen but not vWF or ADP; (2) the adherent platelets release ADP and become sticky; (3) adhesion of platelets in the first milliliter of B to the sticky platelets from A requires vWF and divalent cations but not ADP; (4) retention is maintained thereafter by repetitive platelet-platelet interactions involving ADP release, alteration of adherent platelets by released ADP, and adhesion of further platelets to these ADP-altered platelets which requires vWF.