During latent infection of neurons with herpes simplex virus type 1, viral transcription is restricted to the latency-associated transcripts (LATs). These RNAs contain open reading frames, but detection of a protein encoded by the LATs has not been reported. We used immunocytochemical techniques to demonstrate that an antiserum directed against a bacterially expressed fusion protein containing part of a LAT-encoded polypeptide recognized an antigen present in primary neurons latently infected in vitro. This antigen (called LAA, for latency-associated antigen) was not detected in mock-infected neurons, in productively infected Vero cells, or in neurons latently infected with a mutant virus carrying a deletion in the LAT gene. By Western immunoblot analysis, we demonstrated the presence of a protein with an apparent molecular mass of 80 kDa recognized by the anti-LAA antiserum in latently infected neurons.