It has been reported recently that CD4 is a major component of the receptor for human herpesvirus-7 (HHV-7), which has been newly identified as a T-lymphotropic virus. To investigate further the role of CD4 in HHV-7 infection, we examined the susceptibility to HHV-7 infection of various CD4-negative or weakly positive cell lines into which the cDNA for CD4 was transferred using an adenovirus vector. Out of 13 cell lines transduced with a CD4 gene, one T-lymphoid cell line, one monocytoid cell line, and two myeloid cell lines showed high susceptibility to HHV-7 infection. Taken together with previous studies, these data suggest strongly that CD4 is a major component of the binding receptor for HHV-7.