In ACI/N rats pretreated with cyclophosphamide (CY) growth of the bladder cancer, BC-47, and adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP) activities in lymphocytes were investigated to clarify the possible antitumor effect via the immune system of the chemotherapeutic agent. A single dose of 50 mg/kg of CY with the tumor implantation 3 days later gave rise to tumor regression following temporary progression around day 15 and significant increase of peripheral lymphocytes with higher PNP activity on days 7 to 10 of the tumor implantation. In the thymus such lymphocytes increased 3 days earlier. The antitumor effect was not demonstrated in athymic nude mice. In the light of the results and elimination of suppressor T precursors by CY, it was postulated that T lymphocytes with higher PNP activity act as effector cells in the antitumor immunity whereas suppressor T precursors belong to the cell population with lower PNP activity.