Human intraepithelial lymphocytes (IEL), predominantly CD8+ T lymphocytes located between intestinal epithelial cells, may represent the first-line host defense against colon cancers. This study evaluates their lymphokine-activated killer (LAK) activity and compares it to that of lamina propria lymphocytes (LPL). The phenotypes of the precursor or effector LAK cells were determined by depleting IEL or LPL of specific subsets using antibody and complement lysis either before or after a 7-day culture with interleukin-2 (IL-2), respectively, and then measuring changes in lytic activity. The precursor and effector LAK cells in the IEL were mixed populations with some but not all expressing CD2, CD3, CD8, and NKHl. The LAK activity by LPL was due to CD2+CD3+CD4-CD8- precursor cells and CD8+ effector cells. Since the IEL LAK cells were heterogeneous, their lytic activity against two different target cell types was analyzed. The LAK activity against DLD-1 colonic adenocarcinoma cells was reduced by antibodies to CD2, CD11a, and HML-1, whereas LAK activity against K-562 cells was reduced by antibodies to CD2, CD11a, and CD54, but not HML-1, indicating a target-cell-specific mechanism. Cold competition experiments, however, showed that the same cytotoxic IEL bound to both target cells. In conclusion, LAK activity by IEL is due to a mixed population, whereas that by LPL is due to CD8- precursor cells and CD8+ effector cells. The same effector LAK IEL kill both DLD-1 and K-562 targets, but different mechanisms are involved.