Structure-function studies of human recombinant interferon (IFN) alpha 2c were performed using a panel of specific monoclonal antibodies in the binding and neutralizing assays. Two immunodominant structures, designated sites I and II, were detected and localized within two conserved hydrophilic regions of IFN-alpha molecule. Using the NK2 antibody as a marker, site I was mapped into a carboxy-terminal domain around residues 112-148. This site was shown to be, most probably, responsible for inducing the antiviral and antiproliferative activities of the receptor-bound IFN-alpha 2c in the cell. Site II that mapped into the amino-terminal domain of IFN-alpha 2c was, at least partially, formed by the amino acid residues 36-41. This region was shown to be most probably involved in the binding of IFN to its cellular receptor. These findings fit with Sternberg and Cohen's model (Int. J. Biol. Macromol. 4, 137-144, 1982) for the tertiary structure of human IFN-alpha.