SUMMARY The presence of cellular chimerism in mice rendered tolerant by the neonatal inoculation of homologous lymphohematopoietic cells was assessed by the lymphocytotoxic method and by the use of immune lymphocyte transfer (ILT) reactions. In this latter test, lymphoid cells, isologous to the host but specifically sensitized to the transplantation antigens of the donor strain, were inoculated into the skins of putative chimeras and the injection sites were observed for the development of a delayed inflammatory response. At 100 days or more after the tolerance-conferring inoculum, 70% of test animals were chimeric, as indicated by the ILT reaction, whereas only 20% of similar animals were positive on lymphocytotoxic testing. Evidence is presented in support of the assertion that ILT reactions are extremely sensitive indicators of the presence of chimerism. The elicitation of significant ILT reactions under the conditions of these experiments provides additional support for the “passenger cell” hypothesis, which has reemphasized the important role played by peripheral blood leukocytes in the aggregate histoantigenic stimulus of skin or other solid tissue grafts.