HLA Class I-specific antibodies are usually detected using a flow cytometry HLA crossmatch using T cells. A positive case is shown as a single, right-shifted peak on the anti-IgG FITC histogram of T cells. We report a case showing 2 peaks occurring concurrently, both positive and negative. The positive peak resulted from the binding of HLA Class II-specific antibodies to donor-activated T cells expressing the HLA-DR antigens. This case suggested donor-specific HLA Class II-specific antibodies can bind to some T cells as well as to whole B cells.