CD23 is expressed on a variety of haemopoietic ceils and displays pleiotropic activities in vitro. We report that in addition to CD21 and IgE, CD23 interacts specifltally with the CD1 1 b and CD1 lc, the a chains of the 82 integrin adhesion molecule complexes CD1 1 b-CD18 and CD1 lc-CD18, on monocytes. Full-length recombinant CD23 incorporated into fluorescent liposomes was shown to bind to COS cells transfected with cDNA encoding either CD1 1 b-CD18 or CD1 lc-CD18 but not with CD1 la-CD18. The interaction was specifically inhibited by anti-CD1 1 b or anti-CD1 lc, respectively, and by anti-CD23 MAbs. The functlonal significance of this ligand pairing was demonstrated by triggering CD1 1 b and CDllc on monocytes with either recombinant CD23 or anti-CD1 1 b and anti-CD1 lc MAbs to cause a marked increase in nitrlte-oxidative products and proinflammatory cytokines (IL-l 8, IL-8, and TNFa). These CDSB-mediated activities were decreased by Fab fragments of MAbs to CD11 b, CDllc, and CD23 These results demonstrate that CD1 1 b and CD1 lc are recep tors for CD23 and that thls novel llgand pairing regulates important activities of monocytes.