A sensitive immunoassay for porcine interferon-alpha. 
Two murine monoclonal antibodies (mAbs) directed against different epitopes on recombinant porcine interferon-alpha (IFN-alpha) were selected and used to construct a two-site ELISA.  This ELISA, when performed in a one-step version, detected about 0.5 units ml-1 of IFN-alpha and showed similar sensitivity but better precision than a cytopathic effect inhibition bioassay.  Estimates of IFN-alpha in tissue culture medium by the two assays correlated well.  In contrast, one or several factors in porcine serum reduced the sensitivity of the ELISA.  Measurements of IFN-alpha in porcine serum was, however, possible in a two-step version of the ELISA, with a sensitivity of about 1 unit IFN-alpha ml-1.  Results of ELISA and bioassay agreed, except that the ELISA possibly produced false positive results in two out of a total of 91 sera negative in the bioassay.  In addition, one of 23 sera positive in the bioassay was negative in the ELISA.