We have examined the effect of different monoclonal antibodies (MAb) to the CD11/CD18 family of surface antigens on the production of superoxide anion by phorbol myristate acetate (PMA)-differentiated U-937 cells upon stimulation with concanavalin A (Con A). The anti-CD11c MAb HC1/1 displayed a significant inhibition, whereas MAb to the other members of the CD11 family did not show a relevant inhibitory effect. These findings suggest that the CD11c antigen is involved in the triggering of the respiratory burst response by mononuclear phagocytic cells.