In our earlier papers with murine leprosy, Hawaiian-M (HM) strain was reported to be much less virulent than Hawaiian-B (HB) strain after subcutaneous infection.Then, comparative observations were made in 7 inbred strains (C3H, C3H/He, BALB/c, KK, C57BL/6, DDD and ICR) of mice of both sexes following intraperitoneal infection with HM bacilli and/or HB bacilli, on severity of visceral lesions and survival time of experimental animals.The results indicated that, in general, average survival time was shorter in mice infected with HB bacilli than in mice with HM bacilli and female cases were more resistant to the infection than male cases.In cases infected with 0.5ml (approximately 3×108 bacilli) of a 1: 100 leproma suspension, there was no remarkable difference in average survival time among 5 strains (C3H, C3H/He, BALB/c, KK and C57BL/6) of mice. Average survival time was prolonged slightly in DDD strain mice and clearly in ICR strain mice.However, in cases infected with 0.5ml (approximately 3×106 bacilli) of a 1: 10000 leproma suspension, considerable differences were found in the susceptibility among the strains of mice, as observed in the cases with subcutaneous infection.Especially in famale cases with 3.1×106 HM bacilli, marked differences were observed. Clear prolongation was found in survival time of BALB/c strain mice, compared with that of mice of 3 strains, C3H, C3H/He and KK. Many cases of C57BL/6 strain mice survived throughout the experimental period, although the visceral lesions of these cases were severe. On the other hand, almost all the mice of DDD and ICR strains also survived, showing slight lesions in their viscera at autopsy.Finally, from the above cases intraperitoneally infected with large inoculum size, I would like to point out that developing cell mediated immunity in the host could not generally suppress multiplication of the invaded bacilli in these cases. Therefore, it is not recommended to select a mouse strain adequate for murine leprosy research concerning cell mediated immunity from the results of experiments of intraperitoneal or intravenous infection with the large dose.