Herpes simplex virus (HSV) type 1 strain HFEM, the genome of which contains a deletion of about 4 kilobasepairs (kbp) between 0.7 and 0.8 viral map units, is apathogenic in the tree shrew when the animals are inoculated intravenously, intraperitoneally and/or subcutaneously. Similar results were obtained using Balb/c mice. Studies of the state of viral latency in animals infected with HSV-1 strain HFEM revealed that this strain was unable to colonize the ganglia of tree shrews. Infectious virus could be recovered only from the spleens of latently infected tree shrews. Thus, this system offers new opportunities for investigating the gene functions responsible for the virulence of HSV-1. Marker rescue experiments were performed by the contransfection technique using native DNA of HSV-1 strain HFEM and Bam H I DNA fragment B derived from the pathogenic HSV-1 strain F and cloned in a bacterial vector. A number of different intratypic recombinants were established in which the deleted region of HSV-1 strain HFEM had been repaired. The pathogenicity of these recombinants was examined in vivo. One of the recombinants (HSV-R-HFehx-C19) caused generalized and lethal herpes virus infection in juvenile and adult tupaias, indicating that the virulence of the pathogenic HSV-1 strain F can be transduced by the cloned Bam H I DNA fragment B to the apathogenic HSV-1 strain HFEM.