Antibodies that are specific to the human interferon (IFN)-alpha 1 and -alpha 4 subtypes have been produced by immunizing rabbits with two short synthetic peptides, corresponding to residues 99-111 of IFN-alpha 1 and residues 37-50 of IFN-alpha 4, respectively. The IFN-alpha 1 peptide has at least three closely clustered residues that are different from those in the other IFN-alpha subtypes, while the IFN-alpha 4 peptide has only two unique amino acid residues, separated by five common residues. The antibodies raised against the IFN-alpha 1 peptide react with recombinant human IFN-alpha 1 but do not cross-react with recombinant human IFN-alpha 4 or IFN-alpha 2. The antibodies raised against the IFN-alpha 4 peptide react with IFN-alpha 4, cross-react with IFN-alpha 1 but not with IFN-alpha 2; the affinity of the antibodies to IFN-alpha 1, however, is at least 10 times lower than their affinity to IFN-alpha 4.