To investigate the role ofvpr(viral protein R) in the replication and cytopathicity of human immunodeficiency virus type 1 (HIV-1), infectious proviruses were constructed that were isogenic except for the ability to produce the protein product ofvpr. The experiments described here demonstrate thatvprencodes a 96 amino acid 15 kDa protein. Thevprproduct increases the rate of replication and accelerates the cytopathic effect of the virus in T cells.Vpracts intransto increase levels of viral protein expression. The stimulatory effect ofvpris observed to act on the HIV-1 LTR as well as on several heterologous promoters.