Microinjection of poliovirus RNA in Xenopus oocytes initiates a complete and authentic viral replication cycle that yields newly synthesized infectious virus. This system can be used to study the molecular mechanism of the different steps involved in virus replication. Interestingly, viral replication only occurs if poliovirus RNA is coinjected with factors present in HeLa extracts. We have determined that two HeLa cell factors are required for viral replication in oocytes, one involved in initiation of translation (polio translation factor) and the other in RNA synthesis. Thus, microinjection in oocytes provides a strategy to identify and further analyze the function of these host cell factors and to biochemically dissect the mechanism of initiation of poliovirus translation and RNA synthesis. Here, we review protocols, approaches, and potential issues that can be addressed using the oocyte system.