Injection of the 708-amino-acid (aa) viral protein "large tumor antigen" (T-Ag) of simian virus 40 (SV40) or its N-terminal 272-aa fragment into C57BL/6 (B6) mice (H-2b) primed CD8+ cytotoxic T lymphocytes (CTL) in vivo. Surprisingly, injection of this nonstructural viral protein (or its N-terminal fragment) in soluble form (without adjuvants) was as efficient in priming CD8+ CTL in vivo as the infection of B6 mice with the virus SV40. CTL activated in vivo by immunization with T-Ag proteins or SV40 infection specifically lysed syngeneic RBL5 cells transfected with a T-Ag-encoding vector; these RBL5/M7 transfectants efficiently presented N- and C-terminal T-Ag epitopes in association with H-2 class I restriction elements. N- and C-terminal T-Ag epitopes were recognized by CTL primed in vivo by immunization with the complete T-Ag protein or by infection with SV40, and (as expected) only N-terminal T-Ag epitopes were recognized by CTL primed in vivo by the soluble N-terminal T-Ag fragment. In CD8+ CTL populations primed in vivo by immunization with the complete T-Ag protein or by SV40 infection and restimulated in vitro with RBL5/M7 transfectants in a mixed tumor cell-lymphocyte culture (MTLC), CTL with specificity for C-terminal T-Ag epitopes were selectively expanded in vitro for months. Hence, the in vitro expansion of CTL population with heterogenous recognition specificities can dramatically distort the picture of its specific recognition repertoire primed in vivo.