Acquired tolerance of skin homografts may be brought about experimentally by the introduction of the antigenic stimulus, in the form of living homologous tissue cells, into the embryo before its immunological defence mechanism has become functionally mature (Billingham, Brent & Medawar 1953, 1956). In practice, this may be accomplished in one or other of the following ways, depending on the species concerned: (a) in mice, rats and rabbits, by direct injection of cells into the foetus; (b) in birds, by the injection of blood into the chorioallantoic circulation, or (c) by the parabiosis of embryos, an ingenious technique devised by Hašek (I953) which leads to an exchange of blood cells, or (d) by the transplantation of tissues to the chorioallantoic membrane. Although any one of these techniques is capable of inducing a very high degree of tolerance in respect of skin homografts transplanted in later life each, unfortunately, has its own technical shortcomings. In particular, these techniques are all severely restricted by a very high rate of mortality which is a direct result of experimental interference in utero or in ovo. For this and other reasons they do not easily lend themselves to the analysis of problems which require the use of relatively large numbers of tolerant animals.