The evolution of strategies at the National Cancer Institute (NCI) illustrates the changes in screening that have resulted from advances in cancer biology. The Developmental Therapeutics Program (DTP) operates a tiered anti-cancer compound screening program with the goal of identifying novel chemical leads and biological mechanisms. The DTP screen is a three phase screen which includes: an initial screen which first involves a single dose cytotoxicity screen with the 60 cell line assay. Those passing certain thresholds are subjected to a 5 dose screen of the same 60 cell-line panel to determine a more detailed picture of the biological activity. A second phase screen establishes the maximum tolerable dosage and involves in vivo examination of tumor regression using the hollow fiber assay. The third phase of the study is the human tumor xenograft evaluation.