Lymphotoxin is a protein with a MW of 45,000 daltons derived from activated lymphocytes that kills target cells nonspecifically. Kinetic studies indicate that there is a lag period of about 4 hours before cytotoxicity becomes apparent, even at high concentrations of lymphotoxin. Therefore, the role of lymphotoxin in cell-mediated cytotoxicity would be restricted to situations in which more rapid mechanisms are not operative. It has found that lymphotoxin increases the rate of 45Ca++ uptake by the mouse L-cells used as targets. This effect and the cytotoxicity are abrogated by ouabain. A lymphotoxin-resistent L-cell mutant did not display the 45Ca++ uptake effect. It is not known whether the Ca++ effect is primary or secondary. Neutralization experiments with anti-lymphotoxin have indicated that there are at least two distinct pathways by which immune lymphocytes can destroy target cells in vitro--one that involves secretion of a nonspecific soluble factor, i.e., lymphotoxin, and another that probably requires intimate contact between the plasma membranes of the target and killer cells. This "membrane contact" mechanism may involve formation of channels in the target cell membranes. The transmembrane channel concept is a working hypothesis that is based on experiments by Henkart and Blumenthal in which it was found that antibody and lymphocytes jointly produce ion-conducting channels in planar bilayers of "oxidized cholesterol." In order to supplement and extend this approach we have made an exploratory study of 86Rb+ and 51Cr marker release from lecithin/cholesterol/dicetyl phosphate liposomes by antibody and nonadherent mouse spleen cells. Evidence is presented indicating that the antibody and cells cause direct synergistic marker release from liposomes into the fluid medium. This indicates that they have the capacity to damage phospholipid bilayers. Hence, it seems worthwhile to conduct further studies of the liposome model in order to uncover the mechanism of membrane damage and to assess its relevance to cell-mediated cytotoxicity.