Ethylcarbodiimide (ECDI) couples soluble antigens (Ag) to lymphoid cells bestowing tolerizing potential. We examined whether ECDI-treated allogeneic dendritic cells (DC) could promote Ag-specific T cell unresponsiveness and prolong graft survival. Exposure of murine myeloid DC to ECDI did not affect surface immunophenotype but reduced their ability to cluster with T cells, enhanced their apoptotic death, and markedly reduced their allostimulatory activity. Anti-donor proliferative and cytotoxic T cell responses of mice primed with ECDI-treated DC were markedly inhibited. Secretion of both Th1 (IFNgamma) and Th2 cytokines (IL-5, IL-10) was suppressed. Cardiac allograft survival in mice preconditioned with a single injection of ECDI-DC was prolonged significantly. These results indicate that ECDI-treated DC promote T cell unresponsiveness to donor alloAgs and prolong transplant survival. The effects are not associated with sparing of Th2 responses, but may reflect inhibitory effects of apoptotic donor DC on host immune reactivity.