Serum concentrations of soluble L-selectin by far exceed those of other soluble adhesion molecules, and serum soluble L-selectin concentrations are remarkably stable upon prolonged storage. We present evidence for Ca(2+)-dependent binding interactions between human serum amyloid P (SAP), a proteolysis-resistant pentraxin glycoprotein, and L-selectin, as shown by surface plasmon resonance measurements, protein band shift assays in a native PAGE system, and after SDS-PAGE and membrane transfer. Monoclonal antibodies to L-selectin strongly reduced binding of biotinylated SAP to L-selectin-IgG chimeras immobilized on microtiter plates. As binding was reduced by prior glycopeptidase F treatment of L-selectin but not of SAP, it appears to be based on SAP lectin domain interactions with N-linked L-selectin carbohydrates. In freshly prepared human lymphocytes, SAP incubation induced expression of a beta2 integrin neoepitope associated with high-affinity binding. This was partially blocked by pre-incubation with Fab fragments of two anti-L-selectin antibodies. In flow chamber experiments, SAP inhibited the adherence of human neutrophils to activated endothelium under shear stress. Thus, SAP binds to human L-selectin and affects L-selectin-dependent leukocyte-endothelial interactions.