Nasal swabs and lung samples from pigs experimentally infected with H1N1 swine influenza virus (SIV) were examined for the presence of SIV by the indirect fluorescent antibody assay, immunohistochemistry, cell culture virus isolation, egg inoculation, and 2 human enzyme immunoassays (membrane enzyme immunoassay, microwell enzyme immunoassay). Egg inoculation was considered to be the gold standard for assay evaluation. The 2 human enzyme immunoassays (EIA) and egg inoculation agreed 100% for the prechallenge nasal swabs. Agreement on SIV identification in nasal swabs with egg inoculation following challenge was considered to be good to excellent for membrane EIA (kappa = 0.85) and microwell EIA (kappa = 0.86). Agreement on SIV identification in lung tissue with egg inoculation following challenge was good to excellent for membrane EIA (kappa = 0.75), fair for microwell EIA, fluorescent antibody, and cell culture virus isolation (kappa = 0.48, 0.64, 0.62, respectively), and poor for immunohistochemistry (kappa = 0.36). No assay was 100% accurate, including the "gold standard," egg inoculation. In light of this information, it is important to consider clinical signs of disease and a thorough herd history in conjunction with diagnostic results to make a diagnosis of SIV infection.