Polymorphonuclear leucocyte function was investigated in 19 patients with active Behçet's disease. Spontaneous free leucocyte migration was found to be significantly reduced, yet after stimulation the leucocyte's chemotactic activity was considerably increase (p less than 0-05) when compared to control leucocytes. Control leucocytes migrated more rapidly when incubated in serum taken from patients with Behçet's disease (p less than 0-005). The enhanced chemotactic activity in Bechçet's disease appears to be due to both serum and intrinsic leucocyte factors. Spontaneous nitroblue tetrazolium reduction was found to be normal, although after stimulation leucocyte nitroblue tetrazolium reduction was lower than in the control group (P less than 0-025), as was leucocyte oxygen utilisation. It is suggested that the hyperreactive cellular inflammatory response that characterises Behçet's disease may be due to increased chemotactic activity and minor alterations in functional metabolic activity of leucocytes.