HERPES simplex virus type 1 (HSV-1) Us11 protein, a true late gene product packaged within the virion, is delivered into cells after infection, exhibits a nucleocytoplasmic localization at early times, and later accumulates in the nucleoli(1-5). This RNA-binding basic phosphoprotein, capable of oligomerization, is supposed to be involved in post-transcriptional regulation of gene expression after HSV-1 infection(6,7). Expression of human T-cell leukaemia/lymphoma virus type-I (HTLV-I) and of human immunodeficiency virus type 1 (HIV-1) is post-transcriptionally regulated by Rex and Rev, respectively(8). These proteins are required for the cytoplasmic expression of unspliced gag-pol and singly spliced env transcripts(9,10). Here we show that HSV-1 Us11 protein is able to bind Rex- and Rev-responsive elements and to transactivate envelope retroviral glycoprotein expression.