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In the cell membrane complement receptor 3 (CR3) consists of one alpha chain (CD11b) and one beta chain (CD18). CR3 participates in many immunological processes, especially those involving cell migration, adhesion, and phagocytosis of complement-opsonized microbes. Recent findings of soluble CR3 in body fluids and in culture supernatant from experiments in vitro point to the involvement of ecto domain shedding as a part of the CR3 biology. To monitor such shedding on a quantitative basis, we have developed time-resolved immunofluorometric assays (TRIFMA) to detect soluble CD11b and CD18 in plasma or serum of either human or murine origin. Compared with most enzyme-linked immunosorbent assays methodologies, TRIFMA possesses prominent advantages, including better dynamic range and reproducibility. These assays may contribute to the understanding of the role of shedding of CR3 and other cell adhesion molecules in human disease and animal models involving inflammation.