To evaluate the role of the cytoplasmic tails of CD11b and CD18 in ligand binding and internalization of the human complement receptor type 3 (CR3, CD11b/CD18), wild type, and truncation mutants of CD11b and CD18 cDNA were expressed in COS fibroblastoid cells and assayed for surface membrane expression, ligand binding, and internalization. Truncated CD11b (alpha 1119T) and CD18 (beta 728T) subunits retaining, respectively, two and five residues of the putative cytoplasmic tails were generated by in vitro mutagenesis. Binding of alpha 1119T beta and alpha beta 728T heterodimeric receptors to iC3b was increased by 330% and 210%, respectively, relative to wild type CR3, suggesting that both cytoplasmic tails of CR3 negatively control receptor avidity. Internalization of antibody cross-linked wild type CR3 occurred predominantly through coated pits. Truncation of the cytoplasmic tail of CD18 but not of CD11b markedly reduced coated pit internalization of CR3, an effect also produced by a single F754A mutation involving one of two putative NPXF internalization signals in CD18.