Turkey and chicken interferon-gamma (IFN-gamma) share high identity (96.3% and 97% at the nucleotide and amino acid level, respectively). As such, we predicted that they would be functionally cross-reactive. To test this hypothesis, we produced recombinant turkey and chicken IFN-gamma, and compared their biological properties. Recombinant turkey and chicken IFN-gamma both induce HD11 cells (a chicken macrophage cell line) and LSTC-IAH30 cells (ALV-J-transformed turkey macrophages) to produce nitric oxide (NO), as measured in an avian IFN-gamma bioassay. Polyclonal and monoclonal antibodies, capable of neutralising the effect of chicken IFN-gamma on HD11 cells, were also shown to inhibit the activity of turkey IFN-gamma on these cells. The antibody neutralisation effect on both turkey and chicken IFN-gamma was shown by a significant reduction in NO production by HD11 cells when the neutralising antibodies were present in the bioassay. FACS analysis showed that HD11 and LSTC-IAH30 cells share some cell surface markers.