Thrombin is a chemotaxin for polymorphonuclear leucocytes. In this paper the binding kinetics of ¹²⁵I-labelled thrombin to purified polymorphonuclear leucocytes is characterized. At 4 °C, the ¹²⁵I-labelled thrombin bound to the cells with a half-time of about 3 min. About 0.77 of the bound tracer dissociated upon addition of a surplus of unlabelled thrombin or hirudin. Each cell possessed about 50 receptors with a K_d of 18 pmol/1 and about 6000 receptors with a K_d of 31 nmol/1. Although the binding affinity at 37 °C was 7-10-fold lower than that measured at 4 °C, it may be high enough to constitute the molecular basis for the reported thrombin-induced chemotaxis.