The use of cell culture systems to assess the toxicity of anticancer agents began over 50 years ago following the observation of the antineoplastic effects of nitrogen mustard (1). There are a wide variety of assays designed to evaluate cellular drug sensitivity described in the literature. These assays essentially fall into two groups; those that measure cell survival and those that measure cytotoxicity. Cytotoxicity assays include methods such as trypan blue dye exclusion, ⁵¹Cr release and ³H-thymidine incorporation (2–4) and these assays assess the structural integrity and metabolic function of the cells following drug exposure. In contrast, cell survival assays measure the end result of these effects on the cell which can be either cell death or recovery. A cell survival assay thus requires a measure of the ability of cells to proliferate and this is usually an estimate of the ability of individual cells to form colonies. However, cytotoxicity assays can also measure the ability of cells to proliferate if the cells are allowed a period of growth following drug exposure. This recovery time is comparable to the time taken for formation of colonies in a clonogenic assay.