The dimeric transcription factor CP2 binds a sequence element found near the transcription start site of the human immunodeficiency virus (HIV-1) long terminal repeat. Several groups have suggested that cellular factors binding this element might play a role in modulating HIV-1 promoter activity in vivo. For example, induction of latent HIV-1 gene expression in response to superinfection by herpes simplex virus type 1 (HSV-1) or cytomegalovirus is thought to be mediated, in part, by factors binding the CP2 site. In this report we began to examine directly the relationship between CP2 and expression of the HIV-1 promoter. First, we tested what effect HSV-1 infection of T cells had on the cellular levels of CP2. The results showed that HSV-1 infection led to a significant reduction in the level of CP2 DNA binding activity and protein within 20 h. Next, we tested the effect of overexpressing either the wild-type factor or a dominant negative variant of CP2 on HIV-1 promoter activity in vivo. The results showed that CP2 had little effect or slightly repressed HIV-1 promoter activity in vivo. In addition, these expression constructs had little effect on the induction of HIV-1 promoter activity elicited by HSV-1 infection.