Antibodies (Abs) that mediate antibody-dependent cellular cytotoxicity (ADCC) activity againstHIV-1 are of major interest. A widely used method to measure ADCC Abs is the rapid andfluorometric antibody-dependent cellular cytotoxicity (RFADCC) assay. Antibody-dependentkilling of a labelled target cell line by PBMC is assessed by loss of intracellular CFSE but retention ofmembrane dye PKH26 (CFSE-PKH26+). Cells of this phenotype are assumed to be derived fromCFSE+PKH26+ target cells killed by NK cells.We assessed the effector cells that mediate ADCC in this assay. Backgating analysis andphenotyping of CFSE-PKH26+ revealed that the RFADCC assay's readout mainly representsCD3-CD14+ monocytes taking up the PKH26 dye. This was confirmed for 53 HIV+plasmapurifiedIgG samples when co-cultured with PBMC from three separate healthy donors.Emergence of the CFSE-PKH26+monocyte population was observed upon co-culture of targetswith purified monocytes but not with purified NK cells. Image flow cytometry and microscopyshowed a monocyte-specific interaction with target cells without typical morphologicalchanges associated with phagocytosis, suggesting a monocyte-mediated ADCC process.We conclude that the RFADCC assay primarily reflects Ab-mediated monocyte function.Further studies on the immunological importance of HIV-specific monocyte-mediated ADCCare warranted. © 2012 Elsevier B.V. All rights reserved.