The live attenuated vaccines against poliomyelitis developed by Sabin are among the safest and most effective antiviral vaccines in current use and have eliminated poliomyelitis as a public health problem in many countries. They form the main basis of the WHO intention to eliminate poliomyelitis from the world by the year 2000, and the molecular basis of their attenuated phenotype and some of its virological consequences are increasingly clear. Nonetheless, the data reviewed here show how poorly understood their mechanism of action is in use. Our studies raise the possibility of in vitro neurovirulence tests and may help to identify features of particular importance in the attenuation of the virus for human rather than simian recipients. On the other hand it is clear that when a live virus is used as a vaccine it is not possible to control it in the same way that genetically engineered products may be controlled in so far as replication in the recipient makes it possible for the live vaccine strain to alter in ways which may or may not be undesirable.