DURING the last few years, it has become clear that normal individuals have a lymphocyte subpopulation which has the capacity to kill certain in vitro propagated cell lines^1–3. The effector cells involved in this kind of cytotoxicity have been designated either natural or spontaneous (SK) killer cells and have mainly been defined using lymphocytes derived from the peripheral blood. It has been shown that the majority of SK cells have receptors for the Fc part of IgG, and that some have receptors for the activated third complement factor (C3), but little is known about the specificity, recognition and effector mechanisms underlying spontaneous cytotoxicity^1,3–4. Although the actual biological significance of SK cell cytotoxicity is unclear, it has been proposed that these killer cells act in vivo as surveillor cells against tumour development. We give here evidence in support of this idea, for we have found that SK cells, in a short-term cytotoxicity assay, preferentially kill cell lines derived from leukaemic tumours as compared to cell lines from normal lymphocytes.