WE have established¹ that long-lasting, strain-specific unresponsiveness to H-2 incompatible skin allografts can be induced in 30–50% of adult CBA mice by pretreatment with a single injection of a crude extract of liver from the graft donor strain, combined with a dose of Bordetella pertussis vaccine and a short course of antilymphocyte serum (ALS). Lymphoid cells removed from mice bearing healthy well-established allografts were reactive against graft donor strain antigens when tested in mixed lymphocyte culture (MLC), cell-mediated lympholysis tests (CML) and in graft versus host assays (GvH)². Haemagglutinating and cytotoxic antibodies specific for the donor strain were invariably absent from the serum of unresponsive mice although small quantities of IgGl antibodies have been detected by a more sensitive technique³. We have so far been unable to detect any strain-specific suppressive activity in sera using MLC, CML and GvH tests² or when passively transferring sera to skin-grafted mice.