A 30 % burn injury was found highly immunosuppressive in mice by means of two in vivo measurements of cell-mediated immunity, and this immunosuppression could be prevented by early excision and grafting. FK506, a new immunosuppressive agent, was given at different doses for 12 days after early excision and grafting following burn and all doses prolonged the acceptance time of allografts from 14 to 20 days. Allograft rejection was not seen when animals were on the drug. Higher doses prolonged the rejection more, even after the cessation of the drug, and they caused some degree of immunodepression. Immunosuppressive treatment with FK506 when used following burn injury affected cell-mediated immunity minimally to moderately when compared to burned control groups.