Activated platelets augment THP-1 rolling and adhesion. Addition of 1 to 50 TRAP-activated platelets per THP-1 cell showed a steep bell-shaped dose-response curve of monocyte rolling (A) and adhesion (B) on PMA-activated HUVEC, with a narrow peak at 3 platelets per monocyte. Inhibition of monocyte margination was observed at ratios = 10 platelets per monocyte (n = 4, *P < .01 v no platelet control).