CD5 monoclonal antibodies (MAbs) define a 67,000 kd monomeric glycoprotein expressed predominantly by thymocytes, mature T cells and a subpopulation of B cells. CD5 is believed to be an alternative signaling molecules capable of increasing the supply of second messengers and thereby altering the cellular response threshold to other activation stimuli. Human peripheral blood dendritic cells (PBDC) are a circulating component of the immune dendritic cell family, which also includes Langerhans' cells in epithelia and interdigitating cells in the T-cell domains of lymphoid tissues. PBDC comprise less than 1% of the peripheral blood mononuclear cell fraction. They are morphologically, immunophenotypically, and functionally distinct from monocytes. In this study, we report that at least a subpopulation of PBDC react with the anti-CD5 MAbs Leu-1 and UCHT2, which define the two major non-crossblocking CD5 epitopes. In contrast, Langerhans' cells, interdigitating cells, monocytes, and macrophages were uniformly CD5-. These findings suggest that PBDC can express the CD5 molecule. Furthermore, they define an additional feature of many enriched PBDC that distinguishes them from monocytes and certain other mononuclear leukocytes, and may provide insights into their activation pathways.