Fluorescence histograms generated from immunofluorescence detection of immunoglobulin G (IgG) drug-dependent neutrophil antibodies by flow cytometry. Isolated normal donor neutrophils were incubated with patient’s serum and washed; neutrophil-bound antibodies were detected with fluorescent anti-human IgG. Results shown are for serum from a patient exposed to quinine with drug-induced neutropenia incubated with neutrophils showing higher IgG fluorescence (median fluorescence intensity [MdFI] = 315) with quinine (dark histogram) compared with patient’s serum incubated with neutrophils and buffer/no drug (light histogram, MdFI = 59), indicating the presence of quinine drug-dependent neutrophil antibodies. MdFI values are shown for each histogram.