Macrosialin (mouse CD68), a macrophage-specific member of the lysosomal-associated membrane protein family, displays N-linked glycosylation and a heavily sialylated, mucin-like domain. We show that phagocytosis of zymosan by inflammatory peritoneal macrophages potently alters glycan processing of macrosialin in vitro. The phagocytic glycoform is not induced by other forms of endocytosis and depends on particle internalization. Zymosan uptake does not influence macrosialin protein synthesis, but increases the specific incorporation of d-[2-³H]mannose, d-[6-³H]galactose, N-acetyl-d-[1-³H]glucosamine and l-[5,6-³H]fucose by 2–15-fold. The phagocytic glycoform displays increased binding of agglutinins from peanut, Amaranthus caudatus and Galanthus nivalis, whereas binding of the sialic-acid-specific Maakia amurensis agglutinin is slightly reduced. Digestion by N-Glycanase abolishes the incorporation of [³H]mannose label and Galanthus nivalis agglutinin binding activity, but preserves the incorporation of galactose and N-acetylglucosamine and specific lectin binding. We also show that phagocytosis increases the complexity and length of O-linked chains. The data presented highlight the importance of differential glycosylation in the biology of macrosialin, phagosomes and macrophages in general.