Cytotoxic lymphocytes are typically generated from unfractionated suspensions of human lymphocytes by stimulating with heterogeneous APCs and exogeneous growth factors. We have found that human blood dendritic cells can directly stimulate allogeneic human CD8+ T cells to proliferate and express antigen-specific cytotoxic activity. These primary responses, which are accompanied by the release of T cell growth factor(s), are induced in the absence of CD4+ helper T cells and are not inhibited by anti-CD4 mAb. Both antigen-specific CTL as well as nonspecific NK cells can be elicited by dendritic cells. The NK cell response can be depleted at the precursor level by panning with an anti-CD11b mAb, which removes a CD11b+/CD28-, CD16+ subset from the starting CD4- responders. Allogeneic blood monocytes are neither stimulatory nor inhibitory of these primary CD4- MLRs, even though monocytes present alloantigen in such a way as to be recognized as specific targets for CTL that have been sensitized by dendritic cells. The number of CD8+ cells that are blast transformed and express an activated phenotype (i.e., HLA DR/DQ+, CD25/IL-2R+, CD45R-) reaches 30-40% of the culture at day 4-5, the peak of the helper-independent response. We conclude that antigen-presentation by dendritic cells is sufficient in itself to prime cytolytic precursors. We speculate that using dendritic cell stimulators and CD4- responders in MLRs may be more efficient than standard tissue typing approaches for the detection of subtle, but important class I MHC-restricted histoincompatibilities in human transplantation.