Anti-tumor X anti-lymphocyte heteroconjugates augment colon tumor cell lysis in vitro and prevent tumor growth in vivo. Cross-linking an anti-tumor antibody, specific for tumor cell surface antigens, and an anti-lymphocyte antibody, specific for the T lymphocyte receptor complex (TCR/CD3), produces a heteroconjugate that can direct T cells to lyse tumor cells. We tested the ability of anti-tumor X anti-lymphocyte (CD3) heteroconjugates to redirect human peripheral blood lymphocytes (PBLs) to lyse human colon cancer cells in cytotoxicity assays and in a murine colon tumor model. We demonstrated in vitro, that cultured human PBLs alone produced low levels of tumor lysis, but PBLs treated with anti-tumor X anti-CD3 heteroconjugates produced significantly greater tumor cell lysis (P less than 0.0025). Similarly, nude mice injected with LS174T human colon cancer cells and treated with cultured human PBLs and anti-tumor X anti-CD3 heteroconjugates survived significantly longer than saline control mice (P less than 0.01), or mice treated with PBLs alone (P less than 0.01), or heteroconjugates alone (P less than 0.05). F(ab')2 heteroconjugates were equally as effective in prolonging animal survival, but irrelevant heteroconjugates and monoclonal anti-tumor antibodies showed no therapeutic benefit. Anti-tumor X anti-CD3 heteroconjugates may represent an effective approach to tumor-specific cellular immunotherapy. 