Alveolar macrophages (AMø) were examined for CR1 (C3b receptor, CD35), CR3 (iC3b receptor; CD11b/CD18), and CR4 (iC3b receptor; CD11c/CD18) by assays for binding of C3‐opsonized sheep erythrocytes (EC3b or EC3bi) and uptake of specific monoclonal antibodies (mAbs). In AMø isolates from nine normal volunteers, 49% of cells bound EC3b and 71% bound EC3bi. Quantitation of receptors per cell with [¹²⁵I]mAbs showed 8.5 × 10⁴ CR4, 5.1 × 10⁴ CR3, and 2.6 × 10⁴ CR1. With most AMø preparations, CR3 was the major receptor mediating attachment of EC3bi, despite the predominance of CR4 antigens. Anti‐CR3 inhibited EC3bi rosettes by ≥50%, whereas anti‐CR4 blocked rosettes by ≤18%. U937 cells differentiated with phorbol myristate acetate resembled AMø in receptor expression but exhibited almost no CR4‐dependent rosetting. Despite the relative inability of CR4 to mediate EC3bi attachment, AMø ingestion of [⁵¹Cr]EC3bi was blocked by either anti‐CR3 or anti‐CR4. Two lines of evidence indicated that CR3 were more mobile within the membrane than were CR4. Immunofluorescence staining demonstrated patching and occasional capping of CR3, whereas CR4 remained uniformly distributed. This patching and capping of CR3 required the actin cytoskeleton, as it was inhibited by cytochalasin D. Modulation experiments using surfaces coated with anti‐CR3 or anti‐CR4 also showed that CR3 was more mobile than was CR4. However, there was some variation among AMø isolates from different donors. In seven isolates, no CR4 modulation was produced with anti‐CR4, whereas in six other isolates, CR4 was modulated by 66%. Incubation of cells in cytochalasin D increased modulation of both CR3 and CR4 on mAb‐coated surfaces. Cells exhibiting increased mobility of CR4 showed an increased ability to form CR4‐dependent EC3bi rosettes. The data are consistent with the hypothesis that CR3 and CR4 exhibit a variable association with the cytoskeleton that regulates their mobility and function. A relatively mobile subset of CR3 and/or CR4 mediates EC3bi attachment, whereas a relatively immobile subset of CR3 and/or CR4 fails to mediate EC3bi attachment but functions to promote ingestion of EC3bi.