Previous work has shown that a cytotoxic T lymphocyte (CTL) immune response of syngeneic mice immunized with a chemically-induced BALB/c (H-2d) fibrosarcoma was directed against an individual tumour-associated antigen. To see whether this reaction was restricted by products of the major histocompatibility complex (MHC), anti-H-2 alloantisera to K or D antigens were used to interfere with the CTL-mediated immune response. Antisera to Kd but not to Dd antigens inhibited the lytic activity of CTL against fibrosarcoma cells. In addition, the study of the CTL response in F1 leads to P antitumour immunized chimeric mice showed that antitumour cytotoxicity developed only when F1 and parental host shared the Kd region. Both experiments strongly indicate that recognition of the individual tumour-associated antigen of the BALB/c fibrosarcoma is restricted by the products of H-2Kd genes.