Abstract
A cytotoxic assay using isotope labelled target cells has been devised. Cr51 was found to be a very suitable isotope allowing swift labelling and a high degree of specificity. With regard to accuracy the test system compared favourably with ordinary cytotoxic assays using uptake of supravital stains or morphological changes as criteria of cell death. The test is considered to be useful in quantitative studies of antigens and antibodies involved in transplantation immunology.