In order to investigate host-tumor relationship, the effects of pretreatment with nitrogen mustrad N-oxide and strain-specific tumor tissue on spontaneous mammary tumor development were studied. In neither group was the pretreatment effective in preventing tumor develop ment of virgin female C 3 H/He mice. There were no reduction in spontaneous tumor incidence, no retardation in age at which tumor appeared, nor prolongation in survival time following onset of tumor and in total survival time. On the contrary, the development of the consecutive tumor of tumorous mice increased significantly in the group immuntzed with isologous tumor homogenate. Further, tumor-unspecific antibody was estimated by hemagglutination test. The data are discussed on the point of host-tumor relationship.