A comparison of six temperature-sensitive (ts) mutants with the parental wild-type (wt) virus showed that, when 1-week-old BALB/c mice were inoculated intraperitoneally with 300 pfu of mouse passaged virus, the viruses could be broadly categorized into two groups. Two viruses (wt and tsm6) were lethal at this dose (10 and 2 LD50 respectively); animals died within 4-6 days of inoculation and the virus became generalized infecting heart, lung, liver, spleen, kidney, and salivary glands to high titre (> 4.3 log10 pfu/g). In contrast, for viruses (tsm1, tsm3, and tsm4) not lethal at this dose (300 pfu = 0.1 to 0.25 LD50), viral replication was poor (< 3.4 log10 pfu/g) except in the salivary glands (5.6 to 7.5 log10 pfu/g). Mutant tsm5 failed to replicate in any tissue while mutant tsm2, lethal at this dose (300 pfu = 1 LD50), produced levels of virus similar to those found with tsm1, tsm3, and tsm4. Comparison of all viruses at sub-lethal doses (0.1 to 0.25 LD50) did show minor differences in their replication in heart, and in levels of virus and duration of infection in kidney and salivary glands. More marked differences were evident between the viruses in their ability to be reactivated from the latent state during immunosuppression. Wild-type virus was most easily reactivated in that 67% of animals exhibited virus in salivary glands, heart, lung, spleen, and kidney. Mutants tsm1, tsm2, tsm3 and tsm6 could be reactivated but from fewer animals (33%, 33%, 18%, and 38% respectively) and fewer tissues. Mutants tsm4 and tsm5 could not be reactivated. Differences in the ability of the viruses to replicate in the lungs and to cause pneumonitis in intranasally-inoculated immunosuppressed mice were also seen. Although immunosuppression was necessary for the induction of severe pneumonitis, differences in severity of pneumonitis resulted from differences in the ability of the mutants to replicate in the lung in vivo. These different mutants should prove useful for examining the viral and host factors involved in CMV-induced pneumonitis, and for examining mechanisms involved in latency and reactivation.