This current review is based upon data acquired in recent years concerning the characteristics, origin, differentiation conditions and mechanisms of activation of polymorphonuclear basophils and mast cells. It includes study of the physiological functions of these cells which clearly actively participate in certain defence mechanisms of the body (parasitic infestations, tumours), and play a role in the processes of healing and of tissue repair. Finally, basophils and mast cells, the role of which in immediate type allergic reactions is well established, are also believed to be involved in delayed hypersensitivity reactions and could play a notable role in the pathogenesis of rejection of autografts and in numerous inflammatory diseases at a wide range of sites.