Antisera produced against sarcoma I (SaI) cells in C57BL/6 and CBA mice tested for their cytotoxic activity against normal A lymphocytes and SaI tumor cells, using guinea pig or rabbit complement. Whereas high levels of cytotoxic activity against normal A lymphocytes were revealed with both antisera using either complement, only rabbit complement produced significant lysis of tumor cells under the standard conditions of the cytotoxic test (30 min, 37 C). However, upon prolonged incubation, guinea pig complement also produced high levels of lysis of SaI cells with the C57BL anti-SaI, but not with the CBA anti-SaI serum. Because these two antisera contain antibodies directed mainly against the H-2K and H-2D specificities, respectively, these results seemed to suggest that SaI cells are lysed in the presence of guinea pig complement with anti-H-2K antibodies but not with anti-H-2D antibodies. This conclusion was further supported by the use of antisera of restricted specificity prepared by selective absorption of the C57BL anti-SaI serum with CBA or DBA/2 cells. However, a C57BL anti-A serum which did not contain antibodies capable of lysing SaI target cells on prolonged incubation with guinea pig complement did not inhibit their subsequent lysis with the C57BL anti-SaI serum. These findings might suggest that under these modified conditions, mainly tumor-specific antigen-antibody systems are revealed with guinea pig complement, whereas with rabbit complement, under the standard conditions of the cytotoxic test, all antigen-antibody systems are detected.