SUMMARYA comparative study has been made of the prolongation of rat tailskin xenografts in C57BL mice chronically treated with a variety of dosage-timing schedules of anti-thymoeyte serum (ATS) injections. The data obtained indicated that the dosage of ATS and the intervals between injections were much more important than differences in the clay of starting ATS injection prior to grafting. The histological evidence suggests that small lymphocytes were directly involved in the destruction of skin xenografts and that humoral antibodies might also play a role in the process of rejection, particularly in those xenografts which had survived for a long time under chronic treatment with ATS before starting rejection. Histologically, there is little if any difference in the basic pattern of xenograft rejection between the ATS-treated and control animals.