It has been difficult to study JCV replication because of its restricted host range. In this study, JCV replication was examined using different clones in 293 cells. RT-PCR assay revealed that large T antigen expression in cells transfected with IMR-32-adapted JCVs was significantly greater than in those transfected with Mad-1 or CY. DNA replication assay and viral load verified that the IMR-32-adapted JCVs were replication-competent in 293 cells, but not Mad-1 or CY JCVs. These results suggest that a 293 culture system with IMR-32-adapted JCVs may be a useful tool for assessing replication of JCV in vitro.