We have always been fascinated by borderline creatures. Chimeras, hybrids of multiple animals-and sometimes humans-appear repeatedly in mythology across cultures from ancient times to the present. Since the early 1980s, scientists have been creating cross-species chimeras, first combining mouse species that could not interbreed naturally, then moving on to create chimeras from even more distantly related animals such as sheep and goats. Scientists use chimeras to study fundamental processes of life such as pregnancy, fetal development, and the progress of disease. Chimeras allow scientists to perform experiments that would otherwise be impossible. Ancient chimera myths played on our anxieties about the boundary between man and animal. Interspecies chimeras strike the same chords of disgust and fear in some people as these ancient mythical chimeras did. This paper examines the science of chimeras and biological borderlines and the social implications of creatures that challenge accepted and comfortable ideas about the divisibility of the animal and human worlds. Can human-animal chimeras be made? Activists Stuart Newman and Jeremy Rifkin have filed a patent application for human-animal chimeras, such as the humanzee, to protest patents on all life forms. Newman and Rifkin believe chimeras are emblematic of abuses of biotechnology and are on a slippery slope to human cloning and elimination of the distinction between natural and manufactured things. They are not alone in believing scientists should be more concerned about the ethical implications of their work. However, a majority of scientists, bioethicists, and scholars find Newman and Rifkin’s viewpoint extreme. The creation of chimeras between species-groups of animals that definition cannot interbreed-may seem to challenge the historically-shaky biological species concept. Goat and sheep cells can work together in a single healthy organism. Does this undermine the taxonomical boundaries between them? While existing in a confusing zone between species, chimeras do not challenge the biological species concept as directly as may seem. When these chimeras are viable, they demonstrate shared common ancestry through evolution. Because chimeras cannot breed and generate more chimeras, they do not challenge the species concept.