Persistent tolerant infections (PTI) with lymphocytic choriomeningitis (LCM) virus were initiated in mice by inoculation during the first few days after birth. Tolerance resulted in all animals which did not die during the first few weeks following inoculation. Thus, the effects of various variables upon the incidence of tolerance were determined by measuring the mortality following inoculation. It was found that the passage history of the virus used exerted a marked effect upon the incidence of mortality of newborn mice following intracerebral inoculation. Previous intracerebral passage of the virus inoculum resulted in high mortality and almost no surviving tolerant animals, whereas repeated intraperitoneal passage produced a virus which caused a very high incidence of tolerance. Inoculum dose did not markedly affect the induction of tolerance although in general high doses of virus caused lower mortality (greater tolerance). Intracerebral inoculation produced a greater proportion of tolerant animals than intraperitoneal inoculation. Viruses with a high ability to induce tolerance were described as docile, whereas strains with poor tolerance-inducing capacity but causing high mortality were described as aggressive in terms of their ability to cause immunological conflict and death of the host. The bearing of these findings on general virology with reference to the choice of strains used in experimental research is discussed.