The novel macrolide immunosuppressant FK 506 is a powerful and selective anti-T-cell agent which has a similar mode of action to that of cyclosporin. Clinical studies of FK 506 in liver allograft recipients indicate a lower risk/benefit ratio than with cyclosporin, and wider evaluation of FK 506 in transplant recipients is now under way in multicentre, prospective, controlled trials in both Europe and North America.