The aim of the study was to detect the number of latently infected cells with wild type virus and with mutant with reduced reactivation. Using PCR in situ method we established, that the number of cells containing genome of these viruses do not differ especially between them. The number of cells with LAT expression is significantly reduced in the ganglia infected with mutant with impaired reactivation as we showed using in situ hybridization. Based on the observations of other authors, that high expression of LATs takes place in cells with high copy number of viral DNA our results showed, that after infection of mice with mutant with reduced reactivation less cells contain high copy number of viral DNA than after infection with wild type KOS. We suggest that impaired reactivation of an ICP22 mutant occurs as a result of reduced number of cells with high copy number of viral DNA.