IT IS NOW nearly two hundred years since Horace Walpole coined the term serendipity, which he in part borrowed from the title of an old legend, the "Three Princes of Serendip," who, so the story goes, went forth into the world and made discoveries of things they were not in search of through "chance" or "sagacity." The results that we have to report may well be considered as gains from serendipity, for they indirectly derive from attempts started about six years ago to produce glandular gastric cancer in rats by inducing gastric ulcer and instilling methylcholanthrene daily into the stomach through a rubber catheter. No gastric cancers were produced in this manner, but instead from these experiments there developed two useful experimental tools, one, adenocarcinoma of the breast,1 and the other, four types of leukemia. These leukemias include an acute lymphatic2 and a chronic myelongenous (chloro) leukemia,3