Following uniocular topical corneal inoculation with herpes simplex virus type 1 (HSV), 176-fold more virus was recovered by 14-day cultivation in vitro from latently infected ipsilateral trigeminal ganglia (TG) of BALB/c mice than from TG of C57BL/6 mice (p = 0.002). Since these quantitative differences may reflect a difference in the number of latently infected cells or a difference in the ability of virus to replicate in secondarily infected cells during cultivation in vitro, experiments were designed to estimate the actual number of sites of latency.Two to four months after topical corneal inoculation, when no active ocular disease was present, minced TG were digested with 2% collagenase. The dissociated cells were placed on monolayers of vero cells, incubated at 31°C, and observed for cytopathic effect (CPE) for 14 days.Ipsilateral TG from BALB/c mice produced five-fold more foci of infection than TG from C57BL/6 mice (p = 0.007). The number of foci of infection was also dependent upon the dose of virus used to infect the eye. Following infection with high doses of HSV, virus was reactivated from contralateral TG, but in lower numbers than from ipsilateral TG. In vitro studies showed that the replication of virus in ganglia from BALB/c mice was 3–8-fold greater than that in ganglia from C57BL/6 mice.These data support the hypothesis that host genetic factors and the number of infectious particles inoculated influence the number of latently infected cells in the trigeminal ganglion after corneal infection with HSV. The number of infectious virus particles recovered from latently infected tissues by cultivation in vitro depends upon both the number of sites of latency and the ability of host cells to support viral replication.