We examined the blocking ability of poly(vinyl alcohol) (PVA) in enzyme immunoassays by coating polystyrene microtiter wells with PVA of different molecular weights (MW) and percent hydrolysis (%Hyd). Blocking ability was measured by the differences in non-specific binding of an anti-rabbit IgG-horseradish peroxidase conjugate to coated and uncoated wells. PVA with a MW of 124,000-186,000 and > 99 %Hyd was the most effective in suppressing the binding of the conjugate. This PVA at 0.5% (w/v) was significantly better at reducing non-specific binding than commonly used blocking agents and did not interfere with the specific binding of the conjugate to antigen-coated microtiter wells.