The antibody-dependent cellular cytotoxicity (ADCC) properties of a murine monoclonal antibody (MAb), designated D612 (IgG2a), which reacts with human colon carcinomas, was studied using normal human peripheral blood lymphocytes (PBMNC). Although the level of ADCC of PBMNC with D612 varied among different donors, it was 20 to 30 times higher than the lytic activity of control cultures containing isotype-matched control MAb. Incubation of PBMNC with recombinant interleukin-2 (IL-2) resulted in a 2- to 5-fold augmentation in the cytotoxicity of effector cells exposed to MAb. This augmentation was apparent after subtracting nonspecific cellular cytotoxicity from the total cytotoxicity mediated by activated effector cells in the presence of D612. Optimal stimulation of specific ADCC with IL-2 appeared after 24 hr of culture in 500 U/ml of IL-2, resulting in a 3.8 +/- 1.7 fold increase in lytic units. However, stimulation of ADCC was also evident at 10 U/ml of IL-2. Furthermore, antibody dose titrations with untreated and IL-2 activated effectors showed that the threshold dose of MAb needed for efficient ADCC was reduced by 200-fold with IL-2. Depletion of FcR gamma III-positive lymphoid cells markedly reduced D612 ADCC, demonstrating the participation of NK/LAK cells in D612-mediated ADCC. Low levels of ADCC activity were found associated with adherent cells, either untreated or following their activation with gamma-interferon, while D612 was most active with non-adherent effectors. The specificity and ADCC properties of the D612 MAb suggest that it should be considered as a candidate for immunotherapy of colon cancer, particularly when used in combination with IL-2 plus LAK cell treatment.