Antibody-dependent cell-mediated cytotoxicity (ADCC) was studied in 20 gastric and colorectal cancer patients by the microtest plate assay with sheep erythrocytes as the target cells. In cancer patients (with both good and poor prognosis), the peripheral K-cell level showed a significant decrease as compared with that of normal subjects. No meaningful difference of the K-cell level was observed between the good and poor prognosis groups. For the purpose of the follow-up of cancer patients, the determination of the K-cell level was useful only when time-lapse examination were performed in the same case.