The influence of cyclophosphamide (CY) pretreatment upon the development of tuberculin hypersensitivity has been studied during the course of infection of mice with BCG. An enhancement of the DTH response to BCG antigens occurred during the induction phase, whereas a depression of this response occurred at the peak and during the decay phase of sensitization. The development of the early DTH-promoting and of the late DTH-depressing effect of CY was favoured by the use of a supra-optimal dose of BCG. Both these effects were cell-dependent since they could be transferred adoptively to syngeneic recipient mice with sensitized lymphoid cells but not with specific immune sera. Pretreatment with CY favoured the emergence of cells capable of responding in vitro to BCG antigens in the draining lymph nodes of BCG-infected mice. No simple association however, exists between this in vitro lymphocyte transformation response and the DTH response.