Human B lymphocytes latently infected with Epstein-Barr virus (EBV) synthesize very large amounts (5 x 10(6)/cell) of two small nuclear RNAs called EBERs (Epstein-Barr encoded RNAs). These RNAs are of unknown function and, like many RNA polymerase III (Pol III) transcripts, bind the La autoantigen. We have discovered that the EBERs also associate with a second highly abundant host-encoded protein designated EAP (EBER associated protein). Human EAP is a small (14,777 dalton, 128 amino acid) polypeptide that binds both EBER 1 and EBER 2. EAP is also found in association with one or both of two analogous virally-encoded RNAs found in baboon cells infected with herpesvirus papio (HVP). We have devised a purification procedure for EAP and have cloned its cDNA from a human placental cDNA library using amino acid sequence data and the polymerase chain reaction (PCR). The predicted amino acid sequence of EAP shows a strong resemblance (77% identity) to an endodermal, developmentally regulated sea urchin protein called 217 (Dolecki et al., 1988). EAP contains a potential nuclear localization signal and a highly acidic carboxy terminus, but does not display marked similarity to any other RNA binding proteins.