SUMMARYProlonged survival of skin allografts was obtained in rats treated with enhancing serum. The enhancing isoantiserum was produced by hyperimmunizing the same rat (local white) with grafts from different rats of a semi-inbred hooded Lister strain. When (white X hooded Lister)F1rats were used instead of hooded Lister rats as skin donors, the viability was extended to a mean survival time (MST) of 17.27 days. The importance of reducing the specificities against which the enhancing antibodies are directed or of preparing an isoantiserum containing a pool of such antibodies, for obtaining an optimal enhancing activity, is discussed.