ALTHOUGH a great deal of work has been done on the carcinogenic action of methylcholanthrene, its effects on specific organs and tissue systems have not been completely investigated. Among the methods employed are : skin-painting, subcutaneous injection, intra-peritoneal administration as well as injection into the amniotic fluid of mice. The findings reported here are based on observations by another method which has been employed as part of an investigation for determining the strain specificity of certain endocrine organs of mice of the C57 (black), the Strong A and the C3H (Bar) strains of mice. The results which were obtained are striking enough to deserve wider notice even at this stage.