The observation in the 1950s that nitrogen mustard and other toxic chemicals could induce antitumor responses in patients with refractory lymphoma initiated a massive search for active chemotherapeutic agents. The initial observations stimulated a search for new chemotherapeutic agents which might have increased antitumor activity with less toxicity for normal tissues. To aid in the search for these new chemicals and to attempt to distinguish among the many toxic chemicals which might be candidates for clinical studies, the National Cancer Institute, the pharmaceutical industry, and the cancer research laboratories of most Western nations developed systems for "screening" drugs for antitumor activity. Perhaps the most extensive screening