With the aim to assign differences in activity between murine interferon-alpha 1 and -alpha 4 to specific amino acids, we have constructed hybrid genes and analysed the antiviral properties of the corresponding hybrid proteins. The hybrid genes were constructed by means of homologous recombination between the alpha 1 and alpha 4 genes in Escherichia coli. Hybrids in which the N-terminal part is derived from alpha 1 show that two regions have a major effect on the activity: amino acid 10-20 and 55-67. When comparing hybrids with N-terminal alpha 4 sequences, transitions in activity are found in the same regions. Interestingly, the curves for the two sets of hybrids are exactly each others mirror image.