Previously we had reported that human cytomegalovirus (HCMV) induced replication of plasmids containing the SV40 origin of replication in human fibroblasts that were nonpermissive for SV40 and permissive for HCMV DNA replication. The amplification of SV40 origin-containing plasmids was dependent upon the HCMV-induced expression of T-antigen RNA. From previous studies it was determined that cotransfection of cosmids, containing HCMV genomic DNA, could stimulate SV40 DNA replication and T-antigen production. This indicated that the gene products of HCMV responsible for inducing SV40 DNA replication could be determined. In this study we report that the cotransfection of the major IE gene of HCMV alone was sufficient to facilitate the replication of the SV40 origin-containing plasmid. The HCMV IE1 gene product (i) increased expression of T-antigen RNA and protein and (ii) induced SV40 plasmid DNA replication in a T-antigen-dependent manner. The SV40 replication event was not due only to the expression of T-antigen. When the gene coding for T-antigen was placed under control of the Rous sarcoma viral promoter so that T-antigen expression in HEL cells was constitutive, it was not sufficient to replicate the SV40 plasmid in the absence of the HCMV IE1 protein. Therefore, the major IE gene of HCMV was capable of increasing the expression of T-antigen RNA and facilitating the replication of the SV40 origin. We are currently investigating the mechanism responsible for these observations.