The inhibition of the cellular response of C57Bl/6 mice against allogeneic P815 mastocytoma by procarbazine was shown to be uniquely dependent upon time of administration with respect to antigen. If the drug was given 4 or 6 days after antigen, the development of the T cell effectors was inhibited; if it was given earlier (2 hr or 1 or 2 days after antigen), the response was affected less. In contrast to the immunosuppressive effectiveness of delayed administration of procarbazine, the inhibition of the T cell response by daunorubicin was greatest when the agent was given 2 days after antigen, and that by cyclophosphamide, at the concentrations used, was relatively time independent. Under similar conditions the effects of the three agents on the humoral response were found to be less selective. The inhibition of both responses by procarbazine was shown to be dose dependent and relatively independent of the route of administration.