Semiallogenetic radiation chimeras were prepared by injecting heavily irradiated F1 hybrid mice with bone marrow cells from one parental strain; the bone marrow cells were treated with anti-theta serum and complement to remove T cells and injected in large numbers (2 times 10-7 cells). The mice survived in excellent health until sacrifice 6 mo later. Thoracic duct cannulation at this stage showed that the mice possessed normal numbers of recirculating lymphocytes. Close to 100% of thoracic duct lymphocytes and lymph node cells were shown to be of donor strain origin. The capacity of lymphocytes from the chimeras to respond to host-type determinants was tested in mixed leukocyte culture and in an assay for cell-mediated lympholysis (CML). Mixed leukocyte reactions (MLR) were measured both in vitro and in vivo; tumor cells and phytohemmaglutinin-stimulated blast cells were used as target cells for measuring CML. While responding normally to third party determinants, cells from the chimeras gave a definite, though reduced MLR when exposed to host-type determinants. However, this proliferative response to host-type determinants, unlike that to third party determinants, was not associated with differentiation into cytotoxic lymphocytes. No evidence could be found that unresponsiveness in this situation was due to blocking serum factors or suppressor T cells. It is argued that the results support the concept that lymphocytes responsive in mixed leukocyte culture have a different specificity to those exerting cell-mediated lympholysis.