The possibility of obtaining a syngeneic antitumor effect by immunization with normal allogeneic cells was investigated by tests of the lytic activity of peritoneal exudate cells (PEC) of BALB/c mice immunized with lymphoid cells of either a single strain or a pool of six different allogeneic strains on the syngeneic Moloney virus-induced lymphoma YC8 target and on one of its clones designated YC8-D1. Significant cytotoxicity on both targets but not on two other BALB/c lymphomas was obtained with PEC of BALB/c mice singly immunized to the non-H-2-incompatible but H-2-compatible DBA/2 or B10.D2 lymphoid cells. The lack of lysis of YC8 cells by PEC of BALB/c mice immune to B10.A (H-2k,d) suggests that the in vitro killing was restricted by Kd-IEd region products of the major histocompatibility complex. Pool immunization was effective in generating antitumor cytotoxic lymphocytes only when DBA/2 lymphoid cells were included in the pool. The pattern of reactivity of effectors elicited in (BALB/c x DBA/2)F1 and in (BALB/c x B10.D2)F1 mice by immunization with DBA/2 and B10.D2 cells showed that at least two sets of antigens are recognized on YC8 targets, one shared by DBA/2 and B10.D2 tissues and the other expressed by DBA/2 cells only. Cold target blocking experiments indicated that the same effectors recognized non-H-2 antigens of DBA/2 and the cross-reacting YC8 determinants. The antitumor effect was mediated by T-cells, since it was abrogated by treatment of effectors with anti-Thy 1.2 serum plus complement. These data indicate that determinants defined by cytotoxic T-lymphocytes are expressed on the BALB/c lymphoma YC8 and cross-react with non-H-2 antigens of DBA/2 and B10.D2 strains.