Effect of treatment of mice with cyclophosphamide (CY) on the delayed hypersensitivity (DH) response was investigated in C57BL/6 mice. DH to methylated human serum albumin (MHSA) could be enhanced with CY in young mice but not in aged ones. DH enhancement with CY appeared to be due to elimination of suppressor T cells involved in DH. Effector T cells were also sensitive to CY, the damaging effect of CY on these latter cells was, however, transient suggesting the rapid recovery of effector T cells. The overshooting recovery of the effector T cells required the presence of the thymus. It is more probably that there are at least two distinct subpopulations of T cells in DH, effector T cells, and suppressor T cells. The distinction is already apparent in the thymus stage. The suppressor T cells, categorized as a central regulator, seem to be antigen nonspecific and regulate the more effectively the DH in young mice, thus physiological role of these cells in age-associated immune alterations is implicated.