Expression of CD68 and the S100A8/A9 heterodimer in the synovial tissue (ST) of rheumatoid arthritis (RA). Cryostat sections of ST samples from patients with RA were incubated with mouse immunoglobulin (Ig) G1 monoclonal antibody (mAb) against the S100A8/A9 complex or control mAb, followed by incubation with rhodamine-conjugated donkey anti-mouse IgG1 mAb, and were then incubated with fluorescein isothiocyanate-conjugated anti-CD68 mAb. Two-color immunofluorescence confocal images were obtained for CD68 and S100A8/A9 expression (green and red staining, respectively). The two images were superimposed; double-positive cells are shown in yellow. No isotype-matched control mAb was observed. Similar staining patterns were obtained in additional analyses with two synovial tissue samples from different patients.