Specific cell-mediated immunity to SV 40 tumor-specific transplantation antigen (TSTA) in mice undergoing tumorigenesis by syngeneic SV 40-transformed BALB/C cells was investigated by the macrophage migration inhibition (MMI) and transplantation rejection tests. Specific cellular reactivity to SV40 TSTA could be demonstrated in BALB/c mice early after tumor cell inoculation. This activity was no longer detectable during the later stages of tumor growth but was again demonstrable 2 weeks after tumor excision. Addition of an equal number of non-reactive peritoneal exudate (PE) cells from tumor-bearing mice to PE cells from mice immune to SV40 TSTA specifically abrogated the reactivity of the latter cells to soluble SV40 TSTA. When lymphoid cells with blocking activity were cultured in vitro they not only lost their blocking capacity but also regained their reactivity to SV40 TSTA in the MMI test. These findings indicate that tumor-bearing hosts possess lymphocytes specifically sensitized to the TSTA of the tumor and that the specific reactivity of these cells can be regained after culture in vitro.