Two mice of the TM strain, maintained on Purina chow supplemented with refined coin oil plus free fatty acids, in one case, and with refined corn oil without free fatty acids, in the other case, developed primary heart rhabdomyosarcomas. The tumors metastasized to the lungs and the kidney, in the first case, and to the lungs alone in the second case. One mouse of the TM strain, on the diet supplemented with corn oil plus free fatty acids, developed a fibrosarcoma behind the right anterior extremity. Metastases from the tumor were found in the liver, lungs and myocardium. Another mouse of the C57 Bl. strain, injected subcutaneously, in the interscapular region, at the age of 7 days, with a single dose of 30 mug of Delestrogen, developed, at the point of injection, a large rhabdomyosarcoma. At the age of 5333 days, when the animal was killed, metastases from the tumor were found in the intercostal muscles and in the heart. Three mice, one of the TM strain, on a diet supplemented with corn oil and cholesterol, and two of the C57 Br. strain, on a diet supplemented with raw egg yolk (rich in cholesterol) had organized blood clots in the atria (in two mice) and one extending into the ventricle.