Cell-free fractions of a number of transplanted human tumors induce a deformity in newborn hamsters characterized by small size, flattened foreface or microcephalic, domed head, protruding eyes and tongue, absent or abnormal teeth, and fragile bone structure.' The effect is now thought to be the late result of infection by a filtrable agent readily demonstrable in the tissues of the test animals as well as the tumors. The titer of the agent, presumably a virus, and the virulence of the infection increase rapidly with passage and induce severe morphologic changes and death before the deformity is manifest.The agent, which is not dialyzable, has been found in all 8 of the transplanted human tumors tested. It has been found also in tissues of patients having cancer. It has not been obtained, with one possible exception, from preparations of rat, mouse, or normal hamster tissues, nor in a variety of transplantable animal tumors.