We have designed shuttle vectors containing the late region of simian virus 40 (SV40) DNA (coding for the capsid proteins) which could be encapsidated into pseudo-SV40 virions during passage in monkey cells. We describe here the use of these shuttle viruses as helpers for the encapsidation of another shuttle vector into viral particles. Following cotransfection into monkey cells, the efficiency of encapsidation was similar for the shuttle virus and the other plasmid. The amounts of pseudo-SV40 virions recovered from the two vectors reflected the amounts of their DNA present in monkey cells. Thus, the presence of the SV40 late region did not confer any significant advantage for encapsidation. The encapsidation of any shuttle vector into pseudo-SV40 virions is therefore possible and efficient, shuttle viruses constituting an interesting alternative to the use of SV40 as helper in this process.