Mouse CD4+ T cells efficiently develop in fetal pig thymus (FP THY) grafts and repopulate the periphery of T cell and NK cell-depleted, thymectomized (ATX) mice. However, efficient peripheral repopulation of mouse CD8+ T cells does not occur in these mice. We have therefore evaluated the maturation and function of mouse CD8 single positive (SP) thymocytes in fetal pig thymus and liver fragment (FP THY LIV) grafts. Phenotypic maturity, as measured by upregulated expression of TCR, class I MHC, and Qa-2, and downregulated expression of heat stable antigen (HSA) on CD8 SP cells in FP THY grafts, was similar to that in host thymi of euthymic control mice. Cytolytic T lymphocyte (CTL) activity of thymocytes from FP THY grafts was similar to that of thymocytes from host thymi of euthymic mice, indicating that functional maturation of CD8 SP cells had taken place in the grafts. Furthermore, similarly efficient deletion of V beta 5.1/5.2+ and V beta 11+ CD8 SP cells was observed in FP THY grafts as in host thymi of euthymic control mice. Similar percentages of V beta 6, V beta 7, and V beta 8.1/8.2 expressing cells were also detected among CD8 SP cells in FP THY grafts and host thymi of euthymic controls. Together, our results suggest that normal positive and negative selection occurs, and that mouse CD8+ cells can undergo normal functional and phenotypic maturation in FP THY grafts. Thus, other explanations must be sought for the failure of CD8+ cells to repopulate the peripheral lymphoid tissues of ATX, T cell-depleted, pig THY/LIV-grafted mice.