The mouse mammary fatpad is immunologically privileged relative to the subcutis for the transplantation of nonmalignant tissues. Mouse mammary tumors, however, induce tumor-specific immunity in the mammary fatpad. Immunizing tumor cells were injected into the fatpad or subcutis at several time periods before challenge tumor cells were injected into the subcutis on the contralateral side and the time of onset of concomitant immunity was determined. Mice immunized by transplantion of tumor cells into the subcutis became resistant to challenge tumor transplants before mice immunized with tumor cells transplanted into the fatpad. By utilizing a metastatic tumor line which is resistant to thioguanine and ouabain, it was directly demonstrated that tumor cells from a s.c. implant reached the regional inguinal lymph node before tumor cells from a mammary fatpad tumor implant.