An inhibitor of IBV attachment was demonstrated in bovine, swine, chicken and rabbit serum. The inhibitor found in bovine serum was associated with the gamma globulin fraction. The bovine inhibitor was a heterogeneous macromolecule with its sizes ranging from the 7 S to the 19 S globulins as characterized by ratezonal centrifugation and gel filtration. The inhibitory activity was destroyed by sulfhydryl reagents but resisted neuraminidase and acetone treatment and adsorption with kaolin. The activity of inhibitor from all species was reversed with simple sugars such as sucrose, glucose, and xylose. Alpha and beta methylated glucose was not only capable of reversing the inhibitor but it could increase IBV plaque numbers by 50% to 100% when used alone.