The herpes simplex virus type 2 (HG52) deletion variant JH2604 is avirulent (LD50 greater than 10(7) p.f.u./mouse) for mice compared to the parental wild-type virus (LD50 less than 10(2) p.f.u./mouse) and fails to replicate in vivo. JH2604 has a 1488 bp deletion within the 3 kb BamHI v fragment (0 to 0.02 and 0.81 to 0.83 map units) which removes one copy of the 17 bp direct repeat DR1 element of the 'a' sequence and terminates 522 bp upstream of the 5' end of the immediate early gene 1. In vivo selection after transfection of intact JH2604 DNA with the BamHI g (v + u) joint fragment of HG52 results in the isolation of wild-type virus with an LD50 of less than 10(2) p.f.u./mouse. These results show that a 1488 bp sequence within the terminal portion of the genome long repeat region confers neurovirulence on strain HG52.