The role of cell adhesion molecules (CAM) LFA1, ICAM-1, LFA3, VLA1, VLA4, CD29, CD44, and CD56 in tumor-infiltrating lymphocyte (TIL) and natural killer cell (NK)-mediated killing of target cells was studied. Melanoma cell lines and autologous TIL were derived from seven patients with metastatic melanoma, and cytotoxicity assays were done in the presence and absence of monoclonal antibodies (MoAb) to CAM expressed on melanoma cells or TIL. The melanoma cell lines analyzed were all positive for CD29 and LFA3 expression, negative for LFA1 expression, but showed variable expression of ICAM-1, VLA1, VLA4, CD44, and CD56. The effects of anti-CAM antibodies on TIL-mediated melanoma killing fell into three categories: (1) consistent inhibition of TIL-mediated killing was observed when melanoma cells were pretreated with anti-ICAM1 and anti-LFA-3 MoAb or when TIL were pretreated with anti-LFA1; (2) no effect was observed when melanoma cells were pretreated with anti-CD56; or (3) a discreet, but significant, inhibition was observed when target cells were pretreated with anti-CD29, anti-VLA1, anti-VLA4, and anti-CD44. Cytotoxicity was significantly enhanced by pretreatment of target cells with gamma-interferon (gamma-IFN), although gamma-IFN did not augment surface expression of the CAM studied. The NK-mediated killing of K562 cells was blocked by anti-LFA1, anti-CD18, and anti-ICAM, and partially inhibited by anti-CD44 MoAb. Together, these results suggest that several accessory CAM may play a role in regulating cellular cytotoxicity. Because cytotoxicity generally correlated with the level of expression of CAM in melanoma cells, weak CAM surface expression may provide a means for melanomas to escape immune surveillance.