We studied whether the acute-phase protein ₁-acid glycoprotein (AGP) induces rises in [Ca²⁺]_i in neutrophils and sought to identify the corresponding AGP receptor (or receptors). We found that AGP elicited a minimal rise in [Ca²⁺]_i in Fura-2-loaded neutrophils, and this response was markedly enhanced by pretreatment with anti-L-selectin antibodies. (The EC₅₀ value of the AGP-induced Ca²⁺ response was 9 µg/ml.) Activation of phospholipase-C, Src tyrosine kinases, and PI3 kinases proved to be essential for the AGP-mediated increase in [Ca²⁺]_i, whereas the p38 MAPK and SYK signaling pathways were not involved. Furthermore, antibodies against sialic acid binding, immunoglobulin-like lectin 5 (Siglec-5) and oligosaccharide 3`-sialyl-lactose both antagonized the AGP-induced response and caused an immediate increase in [Ca²⁺]_i in anti-L-selectin-treated neutrophils, which indicates a signaling capacity of Siglec-5. We used modified forms of AGP (treated with mild periodate or neuraminidase) to establish the importance of sialic acid residues. The modified forms of AGP caused a much smaller rise in [Ca²⁺]_i than did unaltered AGP. Affinity chromatography confirmed that unchanged AGP, but not neuraminidase-treated AGP, bound to Siglec-5. Our report provides the first evidence for a signaling capacity by AGP through a defined receptor. Pre-engagement of L-selectin significantly enhanced this signaling capacity.—Gunnarsson, P., Levander, L., Påhlsson, P., Grenegård, M. The acute-phase protein ₁-acid glycoprotein (AGP) induces rises in cytosolic Ca²⁺ in neutrophil granulocytes via sialic acid binding immunoglobulin-like lectins (Siglecs).