Chromium-51 is the most commonly used readioactive marker for the labeling of target cells in cytotoxicity assays. The common name for such assays Chromium Release also describes the fundamental method behind the assay procedure. Methods for the precise and accurate quantification of cytotoxicity are of fundamental importance in immunology particularly for the study of tumor and virus cytolysis.
Cytotoxicity assays can be used in order to study either the defense mechanism in a specific patient or to study target cells, depending on whether the effector (or killer) cells are the unknowns. It can also be used for the study of certain drugs on the immune system.
Chromium fulfills the requirement of a good label. It is generally non-toxic (as Na₂CrO₄), it is easily taken up by the cells, it is spontaneously released only in relatively small amounts, its detection is easy and sensitive and it does not change the morphology and characteristics of the target cell.