Forty-two isolates of infectious pancreatic necrosis virus were cloned by plaquing, and the clones were used in neutralization tests with a 5-strain polyvalent serum. Seventy-six percent of the strains tested were completely neutralized, 19 percent were partially neutralized and 5 percent were not neutralized. Data provide a measure of the antiserum's diagnostic effectiveness and indicate that at least one more antigenic type should be included in preparation of a definitive polyvalent serum.