In this unit, methodology is presented for measuring the capacity of human natural killer (NK) or lymphokine‐activated killer (LAK) cells to lyse tumor cell targets. Cytotoxicity of these effector cells is evaluated in a short‐term ⁵¹Cr‐release assay using NK‐sensitive tumor cells as the targets for NK cells or NK‐resistant tumor cells as the targets for LAK cells. In the basic protocol, a generic ⁵¹Cr‐release assay is described in which PBMC, purified NK cells or interleukin 2 (IL‐2)‐activated lymphocyte populations (LAK cells) are utilized as effector cells. Support protocols describe preparation of nonadherent tumor cells, cells obtained from malignant effusions, trypsinized tumor cells from adherent monolayer culture, or freshly isolated tumor cells from surgical specimens. All of these cell types can serve as the ⁵¹Cr‐labeled target cells in the basic protocol. The procedure for labeling target cells from any of these sources with ⁵¹Cr is also provided.