In this study, we test the hypothesis that co-expression of both the complement receptor type 3 (CR3; CD11b/CD18) and Fc gamma receptor type IIIB (Fc gamma RIIIB) (CD16) are sufficient to mediate Ab-dependent phagocytosis. To explore the roles of these receptors in a simple and well-defined in vitro system, stable transfectants of fibroblasts expressing either CR3, Fc gamma RIIIB, or the combination of CR3 and Fc gamma RIIIB were generated. Cells not expressing either receptor, but exposed to the transfection protocol, were used as controls. Cell surface expression of CR3 and/or Fc gamma RIIIB were confirmed by using both flow cytometry and epifluorescence microscopy. The cell lines were analyzed for their ability to bind and internalize opsonized erythrocytes. Cells expressing both CR3 and Fc gamma RIIIB were able to both bind and phagocytose IgG-coated erythrocytes. In contrast, cells expressing CR3 only were able to phagocytose yeast, but not to bind nor phagocytose IgG-coated erythrocytes. Similarly, cells expressing Fc gamma RIIIB only were able to bind IgG-coated erythrocytes, but not to phagocytose either the erythrocytes or yeast. These studies demonstrate that, although CR3 does not participate in Ab-dependent recognition, it can complement the function of Fc gamma RIIIB to effect Ab-dependent phagocytosis. These studies also suggest that one mechanism for glycosylphosphatidylinositol-linked proteins to mediate intracellular functions is through interactions with transmembrane proteins.