Heterologous viruses have been examined for their ability to accelerate the course of infection with the human immunodeficiency virus (HIV) type 1. In this study, ACH-2 cells persistently infected with HIV-1 exhibited augmented HIV-1 replication as a result of superinfection with herpes simplex virus (HSV) type 1. Using HSV-1 mutants with deletions in the genes encoding immediate-early proteins ICP0, ICP4, and ICP27, it was found that ICP0 and ICP27, but not ICP4, were essential for up-regulation of HIV replication. Northern blot analysis showed that this activation of HIV was characterized by an initial rise in the level of the small, subgenomic (2.0 and 4.3 kb) mRNA species, followed by an increase in the level of unspliced genomic (9.2 kb) mRNA. Such a shift in transcriptional phase recapitulates the early-to-late transition seen in single-step growth curves of acute HIV-1 infection. Thus, HSV can activate HIV-1 from latency in ACH-2 cells, this activation of HIV is independent of productive HSV replication since the delta ICP4 deletion mutant is replication-incompetent, and this activation is evident as an increase in the steady-state levels of HIV transcripts.