Mouse leukocyte CR3 (Mac-1, α M β 2 integrin) was shown to function as a receptor for β-glucans in the same way as human CR3. Soluble zymosan polysaccharide (SZP) or pure β-glucans labeled with FITC or 125 I bound in a saturable and reversible manner to neutrophils, macrophages, and NK cells. This lectin activity was blocked by anti-CD11b mAb M1/70 or 5C6 and did not occur with leukocytes from CR3−/−(CD11b-deficient) mice. SZP preparations containing primarily mannose or glucose bound to CR3, and the binding of 125 I-labeled β-glucan to CR3 was competitively inhibited by β-glucans from barley or seaweed, but not by yeast α-mannan. Also, as with human CR3, the lectin site of mouse CR3 was inhibited by α-or β-methylglucoside (but not d-glucose), α-or β-methylmannoside, and N-acetyl-d-glucosamine. Phagocytosis of zymosan and serum-opsonized zymosan was partially inhibited by anti-CR3 and was reduced to< 40% of normal with leukocytes from CR3−/− mice. As with neutrophils from patients with CD18 deficiency, neutrophils from CR3−/− mice exhibited no phagocytosis of particulate β-glucan. SZP or β-glucans primed CR3 of neutrophils, macrophages, and NK cells for cytotoxicity of iC3b-opsonized tumor cells that otherwise did not trigger killing. β-Glucan priming for cytotoxicity was inhibited by anti-CR3 and did not occur with leukocytes from CR3−/− mice. The primed state of macrophage and NK cell CR3 remained detectable for 18 to 24 h after pulsing with β-glucans. The similarity of mouse and human CR3 in response to β-glucans highlights the utility of mouse tumor models for development of therapeutic β-glucans.