The herpes simplex virus 1 US11 gene encodes a site- and conformation-specific RNA binding regulatory protein. We fused the coding sequence of this protein with that of beta-galactosidase, expressed the chimeric gene in Escherichia coli, and purified a fusion protein which binds RNA in the same way as the infected cell protein. The fusion protein was used to generate anti-US11 monoclonal antibody. Studies with this antibody showed that US11 protein is a viral structural protein estimated to be present in 600 to 1,000 copies per virion. The great majority of cytoplasmic US11 protein was found in association with the 60S subunit of infected cell ribosomes. US11 protein associates with ribosomes both late in infection at the time of its synthesis and at the time of infection after its introduction into the cytoplasm by the virion. US11 protein expressed in an uninfected cell line stably transfected with the US11 gene associates with ribosomal 60S subunits and localizes to nucleoli, suggesting that US11 protein requires no other viral functions for these associations.