Site-specific in vitro mutagenesis was used to direct various amino acid substitutions at conserved positions within the sequence of human interferon-alpha 1 (IFN-alpha 1). The antiviral specific activity of IFN-alpha 1, expressed in M13 as a fusion protein [IFN-alpha 1 (phi WT)], could be altered by single amino acid substitutions. The substitution of glycine for tyrosine at position 123 results in a loss of more than 99% of the antiviral specific activity on human cells, but causes no significant change in the antiviral specific activity on primary bovine cells. The tyrosine at position 123 is thus implicated in determining human cell specificity. Based on analysis of IFN-alpha 2, IFN-alpha 1 contains two dulsulphide bridges between cysteine residues 29 and 139 and cysteine residues 1 and 99. IFN-alpha 1 also contains a fifth cysteine residue at position 86. IFN-alpha 1 (phi WT) carrying three serine for cysteine substitutions at positions 1, 86 and 99 retains 23% of the antiviral specific activity of IFN-alpha 1 (phi WT) on human cells. However, the antiviral activity on bovine cells is not significantly affected by this modification. The presence of the disulphide bridge between residues 1 and 99 thus appears to be required for full antiviral activity on human but not bovine cells. A single serine for cysteine substitution at position 29 reduces the antiviral specific activity on both human and bovine cells by some 95%. This data shows that the disulphide bridge between residues 29 and 139 is critical for the antiviral activity of IFN-alpha's.