Disease-free surgical lung specimens from 13 patients with neoplastic or infectious diseases and from three subjects with non-neoplastic, non-infectious pathology were mechanically disaggregated. Natural cytotoxicity was tested against 51Cr-labelled K562 target cells. Unseparated lung cells had little cytotoxicity against K562 cells. Removal of plastic and nylon-wool-adherent cells resulted in cell preparations (morphologically 80% lymphoid) with increased cytolytic activity against K562 but cytotoxicity levels were considerably lower than those of blood lymphocytes tested in parallel. Similar results were obtained when phagocytic adherent cells were removed with carbonyl iron. The NK-resistant murine TU5 and human Raji lines were not affected by lung effector cells. In vitro exposure to partially purified fibroblast interferon enhanced the cytotoxicity of unseparated or non-adherent lung cells. Thus, unlike in mouse pulmonary tissue, low levels of natural cytotoxic activity are associated with the humans lung.