Antibody-dependent cellular cytotoxicity (ADCC) mediated by peripheral blood monocytes was determined in 120 patients who had gastrointestinal tract (GIT), lung and breast cancer, melanoma, or Hodgkin's and non-Hodgkin's lymphoma. Results were expressed in terms of maximum cytotoxicity and cytotoxicity at E:T = 1:10 and were compared with the results obtained in 63 normal subjects. There was a significant decrease in maximal cytotoxicity for both the GIT cancer and the melanoma patient groups, but not for any of the other groups. These differences were not confirmed when results were expressed at low effector: target cell ratios, e.g., cytotoxicity at E:T = 1:10. The relationship between monocyte ADCC and disease extent was examined in those groups with sufficient numbers. Monocyte ADCC was higher in patients with GIT cancer of limited extent than in patients with extensive GIT cancer and in the control group.