ACRIDINE ORANGE has been used for years as a supravital stain for localization of nucleic acids. This dye is relatively nontoxic when given to the living animal, and it causes an intense staining of the ribonucleic acids of the nucleolus and of the desoxyribonucleic acids of the nucleus.1,2Studies were initiated in this laboratory on the uptake of acridine orange and other aminoacridine compounds by tumors in animals. Many of these compounds were screened by Lewis and associates in the late 1940's for cancer chemotherapy properties, and, although the results were not especially striking, gross staining of the tumors was noted following the administration of some of the compounds.3, 4The aminoacridine chemicals are an interesting group; they include antimalarial and antiparasitic drugs such as quinacrine (Atabrine) hydrochloride and diethylaminoethylamino-4-methylthiaxanthone hydrochloride (Miracil D), antibacterial agents (proflavine and acriflavine), and numerous commercial and laboratory dyes. Most of these chemicals are brightly