Immunization of Strain A/Crgl female mice with living tissue and crude cell membrane preparations of recently arisen isogenic spontaneous mammary carcinomas conferred a marked degree of heightened reactivity against a subsequent challenge with the tumors. The heightened reactivity was manifested, under different experimental conditions, by either increased resistance or increased susceptibility to the tumor isografts. Increased tumor susceptibility could be transferred to normal isogenic hosts by means of serum from actively immunized animals and thus resembles the condition of immunologic enhancement. Experience with 1 tumor could evoke enhancement of another, indicating the existence of at least some cross-reacting antigens among spontaneous mammary carcinomas of this strain. Previous experience with living normal tissue or with normal cell membrane preparations of isogenic origin did not confer heightened tumor reactivity.