Conventional dendritic cells (cDCs) comprise a heterogeneous population of cells that are important regulators of immunity and homeostasis. CD1c+ cDCs are present in human blood and tissues, and found to efficiently activate naïve CD4+ T cells. While CD1c is thought to specifically identify this subset of human cDCs, we show here that also classical and intermediate monocytes express CD1c. Accordingly, the commercial CD1c (BDCA-1)+ Dendritic Cell Isolation Kit isolates two distinct cell populations from blood: CD1c+CD14- cDCs and CD1c+CD14+ monocytes. CD1c+ cDCs and CD1c+ monocytes exhibited strikingly different properties, including their differential regulation of surface marker expression, their levels of cytokine production, and their ability to stimulate naïve CD4+ T cells. These results demonstrate that a commercial CD1c (BDCA-1)+ Dendritic Cell Isolation Kit isolates two functionally different cell populations, which has important implications for the interpretation of previously generated data using this kit to characterize CD1c+ cDCs.