In previous papers we have described the presence of the lethal factor which was exerted by the submandibular glands transplanted from adult male donor mice autologously and isologously. The lethal factor was not demonstrated by submandibular grafts obtained from immature male or adult females. We then hypothesized that the lethal factor may be directly related to sexual maturation of male donors and also to an action of testosterone. To clarify this hypothesis, the present experiments were carried out, and the results obtained indicate that the lethal factor is testosterone-dependent. Submandibular grafts obtained from both immature male and adult female donors which received pretreatment with testosterone enanthate exerted a lethal effect on the hosts. Large dosages of testosterone enanthate, which obviously increased the lethal factor level in the submandibular gland, did not kill normal intact adult mice; nor did it increase the mortality of bilaterally submandibular-sialoadenectomized immature mice beyond that of controls without testosterone treatment, when they were not transplanted. Although the site of production of the lethal factor is unknown, we postulate that the submandibular gland is the source.