Antibody-mediated lymphocyte-induced cytotoxicity (K cell activity) was measured in a system consisting of Chang human liver cells, rabbit anti-Chang antiserum, and Triosil-Ficoll purified human lymphocytes. It was concluded that one K cell probably inactivates or kills one target cell, and that the number of active K cells is similar in the peripheral blood of normal and rheumatoid subjects and is about 0-5%.