The chronic administration of Tilorone [(2,7-bis(diethylamino)ethoxy]fluoren-9-one) with antigen seems to prevent the generation of DTH effector cells. The administration of Tilorone proximal to DTH challenge blocks the expression of DTH. By local transfer of DTH effector cells, this latter effect can be shown to be due to the lack of nonspecific inflammatory cells. By the use of both systemic and local transfer systems, it is also shown that specific DTH effector cells from animals treated with Tilorone are unable to circulate in the normal manner. Overall, these experiments suggest that Tilorone does not block the generation of DTH effector cells, but it does greatly alter the expression of DTH. The relation between these effects and the known interferon induction capacity on Tilorone remains unclear.