We previously demonstrated the cytotoxicity of human LAK cells to solid tumors in vitro. The work we reported here investigated the mechanisms involved in killing of solid tumor cells by LAK cells. It was shown that the cytotoxicity of LAK cells was mediated by some factors secreted by LAK cells and effects of direct contact with targets. Following lysis, the nucli of targets were destroyed and fragments of DNA were released into the medium. The cytotoxicity of LAK cells, under some circumtances, was of a positive correlation to the proliferation of themselves.