Synthetic polypeptides corresponding to hydrophilic regions of human interferon gamma (HuIFN gamma) based on the amino acid sequence of HuIFN gamma inferred from its cDNA sequence were used to produce antibodies in rabbits which reacted with the polypeptides and which might also be expected to recognise native HuIFN gamma. Groups of 3 or 4 rabbits were immunised with synthetic polypeptides corresponding to HuIFN gamma amino acid sequences 1-20, 1-59, 24-59, 36-59 and 87-96 which included major hydrophilic domains of the IFN gamma molecule. All the rabbits produced antibodies which recognised the polypeptide immunogen, but to date only 1 of 4 rabbits immunised with polypeptide 24-59 and 1 of 3 rabbits immunised with polypeptide 1-59 have produced antibodies which also recognise native HuIFN gamma. The positively reacting antiserum from the rabbit immunised with polypeptide 24-59 could only be shown to weakly bind to HuIFN gamma, whereas the positively reacting antiserum from the rabbit immunised with polypeptide 1-59 was shown to both weakly bind to HuIFN gamma and weakly neutralise its in vitro antiviral effect. The results so far obtained suggest that the amino acid sequences close to the N-terminus are important for biological activity.