The occurrence of mammary carcinoma in mice and of an epidermoid cyst, following repeated autologous transplantation of mammary tissue has been reported in earlier papers (1). The present report concerns a round-cell sarcoma which appeared following the same procedure. It is uncertain whether this tumor arose from the stroma or interstitial cells of the transplanted mammary tissue or whether, indeed, it may not be a coincidence having nothing to do with the graft.