Dendritic cells (DC) are a subset of leukocytes whose major function is antigen presentation. We investigated the phenotype and function of enriched (95–98.5%) rat DC. We show that both spleen and thymus DC express the natural killer cell receptor protein 1 (NKR-P1) as a disulfide linked homodimer of 60 kD. Freshly isolated DC express a low level of NKR-P1, which is strongly upregulated after overnight culture. Spleen, but not thymus DC, were able to kill the NK-sensitive YAC-1 cell line in vitro, and since this killing was Ca²⁺ dependent, a Fas ligand–Fas interaction was probably not involved. Besides their potent antigen-presenting function, DC can thus be cytotoxic for some tumor targets.