In order to investigate those viruses which are difficult to propagate in tissue culture, it is necessary to develop artificial systems which will fascilliate molecular analysis. One approach has been the development of pseudotype viruses. JC virus (JCV), the etiological agent of progressive multifocal leukoencephalopathy (PML), which is particularly difficult to grow in culture, has been artificially generated as a pseudotype virus (JCPV) by transfecting and expressing the genes coding the viral coat proteins (VP1, VP2, VP3) in COS7 cells under control of the SR alpha promoter. We have analyzed the morphology, function and molecular characteristics of the JCPV. Electron microscopic examination of purified JCPV disclosed a virus structure identical to that of wild-type JCV. In addition, viral DNA could be detected by Southern blot hybridization. The infectivity of JCPV for COS7 cells was demonstrated by PCR, and localization of virus in the nuclei of infected cells was confirmed by in situ PCR. On the basis of these results, it is clear that JCPV is composed of artificial recombinant virus particles possessing infectivity, and that this may be useful not only for studies of viral replication but also perhaps as a vehicle for gene transfer.