A study of the passive transfer of delayed hypersensitivity to DNP-poly-L-lysine and to DNP-GL was performed in Hartley guinea pigs. Delayed hypersensitivity to DNP-PLL and DNP-GL could be transferred successfully only by means of sensitized cells from genetic responder guinea pigs and in most cases, only into those guinea pigs genetically capable of responding to PLL. The inability to transfer delayed hypersensitivity to DNP-PLL or DNP-GL to genetic nonresponder guinea pigs is not the result of the early destruction of the transferred cells by an incompatible host, since it was shown that delayed hypersensitivity to ovalbumin could be successfully transferred from guinea pigs with the PLL gene into genetic nonresponder animals. The requirement of active participation of specific and genetically controlled host mechanisms in the successful passive transfer of delayed sensitivity to DNP-PLL and DNP-GL has been demonstrated.